Structured particles

ABSTRACT

A powder comprising pillared particles for use as an active component of a metal ion battery, the pillared particles comprising a particle core and a plurality of pillars extending from the particle core, wherein the pillared particles are formed from a starting material powder wherein at least 10% of the total volume of the starting material powder is made up of starting material particles having a particle size of no more than 10 microns.

PRIORITY APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/128,365, having a 371(c) date of Feb. 10, 2014, entitled StructuredParticles, which is a national stage entry, under 35 U.S.C. § 371, ofPCT/GB2012/051475, filed on Jun. 22, 2012, entitled StructuredParticles, which claims priority to GB 1110785.1, filed on Jun. 24,2011, the disclosure of each of which is hereby incorporated byreference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates to particles comprising a core and pillarsextending from the core, a method of making said particles and use ofsaid particles in a rechargeable metal ion battery.

BACKGROUND OF THE INVENTION

Rechargeable lithium-ion batteries are extensively used in portableelectronic devices such as mobile telephones and laptops, and arefinding increasing application in electric or hybrid electric vehicles.However, there is an ongoing need to provide batteries that store moreenergy per unit mass and/or per unit volume.

The structure of a conventional lithium-ion rechargeable battery cell isshown in FIG. 1. The battery cell includes a single cell but may alsoinclude more than one cell. Batteries of other metal ions are alsoknown, for example sodium ion and magnesium ion batteries, and haveessentially the same cell structure.

The battery cell comprises a current collector for the anode 10, forexample copper, and a current collector for the cathode 12, for examplealuminium, which are both externally connectable to a load or to arecharging source as appropriate. A composite anode layer 14 overlaysthe current collector 10 and a lithium containing metal oxide-basedcomposite cathode layer 16 overlays the current collector 12 (for theavoidance of any doubt, the terms “anode” and “cathode” as used hereinare used in the sense that the battery is placed across a load—in thissense the negative electrode is referred to as the anode and thepositive electrode is referred to as the cathode).

The cathode comprises a material capable of releasing and reabsorbinglithium ions for example a lithium-based metal oxide or phosphate,LiCoO₂, LiNi_(0.8)Co_(0.15)Al_(0.05)O₂, LiMn_(x)Ni_(x)Co_(1−2x)O₂ orLiFePO₄.

A porous plastic spacer or separator 20 is provided between thegraphite-based composite anode layer 14 and the lithium containing metaloxide-based composite cathode layer 16. A liquid electrolyte material isdispersed within the porous plastic spacer or separator 20, thecomposite anode layer 14 and the composite cathode layer 16. In somecases, the porous plastic spacer or separator 20 may be replaced by apolymer electrolyte material and in such cases the polymer electrolytematerial is present within both the composite anode layer 14 and thecomposite cathode layer 16. The polymer electrolyte material can be asolid polymer electrolyte or a gel-type polymer electrolyte and canincorporate a separator.

When the battery cell is fully charged, lithium has been transportedfrom the lithium containing metal oxide cathode layer 16 via theelectrolyte into the anode layer 14. In the case of a graphite-basedanode layer, the lithium reacts with the graphite to create thecompound, LiC₆. The graphite, being the electrochemically activematerial in the composite anode layer, has a maximum capacity of 372mAh/g. (“active material” or “electroactive material” as used hereinmeans a material which is able to insert into its structure, and releasetherefrom, metal ions such as lithium, sodium, potassium, calcium ormagnesium during the respective charging phase and discharging phase ofa battery. Preferably the material is able to insert and releaselithium.)

The use of a silicon-based active anode material is also known in theart. Silicon has a substantially higher maximum capacity than graphite.However, unlike active graphite which remains substantially unchangedduring insertion and release of metal ions, the process of insertion ofmetal ions into silicon results in substantial structural changes,accompanied by substantial expansion. For example, insertion of lithiumions into silicon results in formation of a Si—Li alloy. The effect ofLi ion insertion on the anode material is described in, for example,“Insertion Electrode Materials for Rechargeable Lithium Batteries”,Winter et al. Adv. Mater. 1988, 10. No. 10, pages 725-763.

WO2009/010758 discloses the etching of silicon powder in order to makesilicon material for use in lithium ion batteries. The resulting etchedparticles contain pillars on their surface. The pillared particles maybe fabricated by etching a particle having an initial size of 10 to 1000microns.

WO 2010/040985 disclosed a method of etching particles having a meanparticle diameter in the range of 5-500 microns.

U.S. Pat. No. 7,402,829 discloses etching of a silicon substrate to forman array of silicon pillars extending from the silicon substrate.

JP 2004281317 discloses growth of silicon nanowires by vapour depositionon a substrate for use in a lithium ion battery anode.

US 2010/0285358 discloses silicon nanowires grown on a substrate for usein a lithium ion battery.

US 2010/0297502 discloses silicon nanowires grown on carbon particlesfor use in a lithium ion battery.

US 2008/0261112 discloses a network of entangled silicon nanowiresconnecting silicon particles for use in a lithium ion battery anode.

WO 2011/117436 discloses a carbon nanofibre including a plurality ofcrystalline whiskers extending from the surface of the carbon nanofibre.

SUMMARY OF THE INVENTION

In a first aspect the invention provides a powder comprising pillaredparticles for use as an active component of a metal ion battery, thepillared particles comprising a particle core and a plurality of pillarsextending from the particle core, wherein the pillared particles areformed from a starting material powder wherein at least 10% of the totalvolume of the starting material powder is made up of starting materialparticles having a particle size of no more than 10 microns.

Optionally, the pillars are formed from a material that, in use,undergoes a volume expansion of at least 10% upon complete insertioninto the material of the metal ions of the metal ion battery.

Optionally, the pillars comprise silicon.

Optionally, the core comprises silicon or carbon.

Optionally, the volume of the pillars is at least 20% of the totalvolume of the plurality of particles, optionally at least 40%.

Optionally, a BET value of the pillared particles is less than 200 m²/g,optionally less than 100 m²/g, optionally less than 60 m²/g, optionallyless than 35 m²/g.

Optionally, an aspect ratio of the particle core is at least 2:1.

Optionally, an average pillar density of the pillars on the particlecore is in the range 10-80%.

Optionally, the mean average pillar diameter is less than 80 nm.

Optionally, opposing surfaces of the particles carry pillars.

Optionally, an average length of the pillars is less than 5 microns,optionally less than 4 microns.

Optionally, only one of two opposing surfaces of the particles carriespillars.

Optionally, an average length of the pillars is less than 10 microns,optionally less than 8 microns.

Optionally, the particles are substantially discrete from one another.

Optionally, at least 50% of the total volume of the starting materialpowder is made up of starting material particles having a particle sizeof less than 15 microns.

Optionally, at least 90% of the total volume of the starting materialpowder is made up of starting material particles having a particle sizeof less than 25 microns.

Optionally, the particle sizes are as measured by a laser diffractionmethod in which the particles being measured are assumed to bespherical, and in which particle size is expressed as a sphericalequivalent volume diameter.

The powder may consist essentially of the pillared particles, or it maybe part of a composition comprising one or more further components.

Accordingly, in a second aspect the invention provides a compositioncomprising a powder according to the first aspect, and at least onefurther component.

Optionally according to the second aspect, the at least one furthercomponent comprises at least one further active component, optionallyactive carbon, optionally graphite.

Optionally according to the second aspect, the at least one furthercomponent comprises at least one conductive, non-active component,optionally conductive, non-active carbon.

Optionally according to the second aspect, the at least one furthercomponent comprises a binder.

Optionally according to the second aspect, the composition has acomposite porosity, as a percentage of the total volume of thecomposite, that is at least the value given by the sum of the volume ofpillars multiplied by 2 and the volume of particle cores multiplied by1.2.

Optionally according to the second aspect, the at least one furthercomponent comprises a solvent.

In a third aspect the invention provides a metal ion battery comprisingan anode, a cathode and an electrolyte between the anode and cathodewherein the anode comprises a powder according to the first aspect or acomposition according to the second aspect.

Optionally according to the third aspect, the metal ion battery is alithium ion battery.

In a fourth aspect the invention provides a method of forming a metalion battery according to the third aspect comprising the step of formingthe anode by depositing a composition according to the second aspect andevaporating the solvent.

In a fifth aspect the invention provides a method of forming a powderaccording to the first aspect comprising the step of etching particlesof the starting material powder to form the pillared particles.

Optionally according to the fifth aspect, the mean average length ofpillars is less than 5 microns.

In a sixth aspect the invention provides a method of forming a powderaccording to the first aspect comprising the step of growing pillars onparticles of the starting material powder.

Optionally according to the sixth aspect, the pillars are grown on onesurface only of the particles of the starting material powder.

In a seventh aspect the invention provides a powder comprising pillaredparticles for use as an active component of a metal ion battery, thepillared particles comprising a particle core and a plurality of pillarsextending from the particle core, wherein at least 10% of the totalvolume of the powder is made up of particles having a particle size ofno more than 10 microns.

The powder of the pillared particles of the seventh aspect may have anyof the optional features described with reference to the powdercomprising pillared particles of the first aspect, including withoutlimitation the material of the pillars and particle core; the volumepercentage of the pillars; the BET values of the powder, the aspectratio of the particles; average pillar density; and size distribution ofthe pillared particles.

The powder of the pillared particles of the seventh aspect may form partof a composition of the powder and at least one further component. Theone or more further components may be as described in the second aspect.

The powder of the seventh aspect, or a composition containing thepowder, may be comprised in the anode of a metal ion battery, optionallya lithium ion battery, as described anywhere in the third aspect. Thismetal ion battery may be formed as described anywhere in the fourthaspect. The powder of the seventh aspect may be formed as describedanywhere in the fifth or sixth aspects of the invention.

In an eighth aspect the invention provides a particle for use as anactive component of a metal ion battery, the particle comprising aparticle core and pillars extending from the particle core, wherein anaspect ratio of the particle core is at least 2:1.

The particle of the eighth aspect may comprise any of the optionalfeatures described in connection with the first aspect of the invention,either alone or in combination.

Particles of the eighth aspect the invention may form a powder ofpillared particles formed from a starting material powder wherein atleast 10% of the total volume of the starting material powder is made upof starting material particles having a particle size of no more than 10microns. In this case, optionally at least 50% of the total volume ofthe starting material powder is made up of starting material particleshaving a particle size of less than 15 microns. Optionally at least 90%of the total volume of the starting material powder is made up ofstarting material particles having a particle size of less than 25microns. Optionally, the particle sizes are as measured by a laserdiffraction method in which the particles being measured are assumed tobe spherical, and in which particle size is expressed as a sphericalequivalent volume diameter. This powder may consist essentially of thepillared particles, or it may comprise one or more further components.

Particles of the eight aspect may form a powder as described anywhere inthe seventh aspect.

Particles of the eighth aspect may form part of a composition comprisingone or more further components as described with reference to the secondaspect.

The anode of a metal ion, optionally a lithium ion battery, may comprisea powder or composition comprising particles of the eighth aspect. Thisanode of a metal ion battery may be formed by depositing saidcomposition in a solvent, and evaporating the solvent.

The particle of the eighth aspect may be formed as described withreference to the fifth aspect or the sixth aspect.

In a ninth aspect the invention provides a particle for use as an activecomponent of a metal ion battery, the particle comprising a particlecore and pillars extending from the particle core, wherein at least onedimension of the particle is less than 10 microns.

The particle of the ninth aspect may comprise any of the optionalfeatures described in connection with the first aspect of the invention,either alone or in combination.

Particles of the ninth aspect the invention may form a powder ofpillared particles formed from a starting material powder wherein atleast 10% of the total volume of the starting material powder is made upof starting material particles having a particle size of no more than 10microns. In this case, optionally at least 50% of the total volume ofthe starting material powder is made up of starting material particleshaving a particle size of less than 15 microns. Optionally at least 90%of the total volume of the starting material powder is made up ofstarting material particles having a particle size of less than 25microns. Optionally, the particle sizes are as measured by a laserdiffraction method in which the particles being measured are assumed tobe spherical, and in which particle size is expressed as a sphericalequivalent volume diameter. This powder may consist essentially of thepillared particles, or it may comprise one or more further components.

Particles of the ninth aspect may form a powder as described anywhere inthe seventh aspect.

Particles of the ninth aspect may form part of a composition comprisingone or more further components as described with reference to the secondaspect.

The anode of a metal ion, optionally a lithium ion battery, may comprisea powder or composition comprising particles of the ninth aspect. Thisanode of a metal ion battery may be formed by depositing saidcomposition in a solvent, and evaporating the solvent.

The particle of the ninth aspect may be formed as described withreference to the fifth aspect or the sixth aspect.

In a tenth aspect the invention provides a powder comprising particleshaving a particle core and pillars extending from the particle core foruse as an active component of a metal ion battery wherein a BET value ofthe particle is less than 200 m²/g, optionally less than 100 m²/g,optionally less than 60 m²/g, optionally less than 35 m²/g.

The particle of the tenth aspect may comprise any of the optionalfeatures described in connection with the first aspect of the invention,either alone or in combination.

Particles of the tenth aspect the invention may form a powder ofpillared particles formed from a starting material powder wherein atleast 10% of the total volume of the starting material powder is made upof starting material particles having a particle size of no more than 10microns. In this case, optionally at least 50% of the total volume ofthe starting material powder is made up of starting material particleshaving a particle size of less than 15 microns. Optionally at least 90%of the total volume of the starting material powder is made up ofstarting material particles having a particle size of less than 25microns. Optionally, the particle sizes are as measured by a laserdiffraction method in which the particles being measured are assumed tobe spherical, and in which particle size is expressed as a sphericalequivalent volume diameter. This powder may consist essentially of thepillared particles, or it may comprise one or more further components.Particles of the tenth aspect may form a powder as described anywhere inthe seventh aspect.

Optionally according to the tenth aspect, a pillar mass fraction PMF ofthe pillared particles is in the range 10-60%, preferably 20-60%,wherein:PMF=[(Total mass of pillars extending from the particle core)/(Totalmass of pillared particle)]×100.

Optionally according to the tenth aspect, a BET/PMF ratio is less than3, optionally less than 2, optionally less than 1.5, optionally lessthan 1, wherein BET is in m2/g.

Optionally according to the tenth aspect, the BET/PMF ratio is less than1.75.

Optionally according to the tenth aspect, the particle cores and pillarshave substantially the same density, and PVF=PMF wherein:PVF=[(Total volume of pillars extending from the particle core)/(Totalvolume of pillared particle)]×100.

Particles of the tenth aspect may form part of a composition comprisingone or more further components as described with reference to the secondaspect.

The anode of a metal ion, optionally a lithium ion battery, may comprisea powder or composition comprising particles of the tenth aspect. Thisanode of a metal ion battery may be formed by depositing saidcomposition in a solvent, and evaporating the solvent.

The powder of the tenth aspect may be formed as described with referenceto the fifth aspect or the sixth aspect.

In an eleventh aspect the invention provides a particle for use as anactive component of a metal ion battery, the particle comprising aparticle core and pillars extending from the particle core, wherein thevolume of the pillars is at least 20% of the total volume of theparticle, optionally at least 40%.

The particle of the eleventh aspect may comprise any of the optionalfeatures described in connection with the first aspect of the invention,either alone or in combination.

Particles of the eleventh aspect the invention may form a powder ofpillared particles formed from a starting material powder wherein atleast 10% of the total volume of the starting material powder is made upof starting material particles having a particle size of no more than 10microns. In this case, optionally at least 50% of the total volume ofthe starting material powder is made up of starting material particleshaving a particle size of less than 15 microns. Optionally at least 90%of the total volume of the starting material powder is made up ofstarting material particles having a particle size of less than 25microns. Optionally, the particle sizes are as measured by a laserdiffraction method in which the particles being measured are assumed tobe spherical, and in which particle size is expressed as a sphericalequivalent volume diameter. This powder may consist essentially of thepillared particles, or it may comprise one or more further components.

Particles of the eleventh aspect may form a powder as described anywherein the seventh aspect.

Particles of the eleventh aspect may form part of a compositioncomprising one or more further components as described with reference tothe second aspect.

The anode of a metal ion, optionally a lithium ion battery, may comprisea powder or composition comprising particles of the eleventh aspect.This anode of a metal ion battery may be formed by depositing saidcomposition in a solvent, and evaporating the solvent.

The particle of the eleventh aspect may be formed as described withreference to the fifth aspect or the sixth aspect.

In a twelfth aspect the invention provides a powder comprising pillaredparticles for use as an active component of a metal ion battery wherein:

the pillared particles comprise a particle core and a plurality ofpillars extending from the particle core; and

a BET/PMF ratio of the pillared particle is less than 3, optionally lessthan 2, optionally less than 1.5, optionally less than 1, wherein:

BET is in m2/g, andPMF=[(Total mass of pillars extending from the particle core)/(Totalmass of pillared particle)]×100.

Optionally according to the twelfth aspect, a BET value of a powder of aplurality of the particles is less than 200 m²/g, optionally less than100 m²/g, optionally less than 60 m²/g, optionally less than 35 m²/g.

Optionally according to the twelfth aspect, a pillar mass fraction PMFis in the range 10-60%, preferably 20-60%.

Optionally according to the twelfth aspect, the BET/PMF ratio is lessthan 1.75.

The powder of the twelfth aspect may comprise any of the optionalfeatures described in connection with the first aspect of the invention,either alone or in combination.

Particles of the twelfth aspect the invention may form a powder ofpillared particles formed from a starting material powder wherein atleast 10% of the total volume of the starting material powder is made upof starting material particles having a particle size of no more than 10microns. In this case, optionally at least 50% of the total volume ofthe starting material powder is made up of starting material particleshaving a particle size of less than 15 microns. Optionally at least 90%of the total volume of the starting material powder is made up ofstarting material particles having a particle size of less than 25microns. Optionally, the particle sizes are as measured by a laserdiffraction method in which the particles being measured are assumed tobe spherical, and in which particle size is expressed as a sphericalequivalent volume diameter. This powder may consist essentially of thepillared particles, or it may comprise one or more further components.

Particles of the twelfth aspect may form a powder as described anywherein the seventh aspect.

Particles of the twelfth aspect may form part of a compositioncomprising one or more further components as described with reference tothe second aspect.

The anode of a metal ion, optionally a lithium ion battery, may comprisea powder or composition comprising particles of the twelfth aspect. Thisanode of a metal ion battery may be formed by depositing saidcomposition in a solvent, and evaporating the solvent.

The particle of the twelfth aspect may be formed as described withreference to the fifth aspect or the sixth aspect.

In a thirteenth aspect the invention provides a composite electrodelayer comprising electroactive pillared particles comprising a particlecore and a plurality of pillars extending from the particle core whereinthe composite electrode expands by less than 150%, preferably less than125%, when charged for a first time to 3,000 mAh/g, the capacity beingper gram of electroactive material in the composite electrode.

Optionally according to the thirteenth aspect, electrode thicknessexpansion upon charging for the first time to 2,000 mAh/g is less than60%, more preferably less than 50%.

Optionally according to the thirteenth aspect, electrode thicknessexpansion upon charging for the first time to 1,500 mAh/g is less than35%, more preferably less than 30%.

Optionally according to the thirteenth aspect, the electroactivepillared particles are silicon electroactive pillared particles.

Optionally according to the thirteenth aspect, the composite electrodefurther comprises one or more materials selected from binders, furtherelectroactive materials and non-electroactive conductive materials.

Optionally according to the thirteenth aspect, the pillared particlesare the only electroactive material in the composition.

The particles of the composite electrode of the thirteenth aspect maycomprise any of the optional features described in connection with thefirst or second aspect of the invention, either alone or in combination.

Particles of the thirteenth aspect the invention may form a powder ofpillared particles formed from a starting material powder wherein atleast 10% of the total volume of the starting material powder is made upof starting material particles having a particle size of no more than 10microns. In this case, optionally at least 50% of the total volume ofthe starting material powder is made up of starting material particleshaving a particle size of less than 15 microns. Optionally at least 90%of the total volume of the starting material powder is made up ofstarting material particles having a particle size of less than 25microns. Optionally, the particle sizes are as measured by a laserdiffraction method in which the particles being measured are assumed tobe spherical, and in which particle size is expressed as a sphericalequivalent volume diameter. This powder may consist essentially of thepillared particles, or it may comprise one or more further components.

Particles of the thirteenth aspect may form a powder as describedanywhere in the seventh aspect.

Particles of the thirteenth aspect may form part of a compositioncomprising one or more further components as described with reference tothe second aspect.

The anode of a metal ion, optionally a lithium ion battery, may comprisea powder or composition comprising particles of the thirteenth aspect.This anode of a metal ion battery may be formed by depositing saidcomposition in a solvent, and evaporating the solvent.

The particle of the thirteenth aspect may be formed as described withreference to the fifth aspect or the sixth aspect.

In one embodiment according to any of the aforementioned aspects, thecore may be an active graphite core. The core may be active graphene,for example a graphene core of a pillared particle as described in anyone of the eighth, ninth, tenth, eleventh, twelfth or thirteenth aspectsof the invention.

DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to thedrawings wherein;

FIG. 1 is a schematic illustration of a lithium ion battery;

FIG. 2A illustrates schematically a pillar of a pillared particleaccording to an embodiment of the invention;

FIG. 2B illustrates schematically a pillar of a pillared particleaccording to an embodiment of the invention;

FIG. 2C illustrates schematically a pillar of a pillared particleaccording to an embodiment of the invention;

FIG. 2D illustrates schematically a pillar of a pillared particleaccording to an embodiment of the invention;

FIG. 2E illustrates schematically a pillar of a pillared particleaccording to an embodiment of the invention;

FIG. 2F illustrates schematically a pillar of a pillared particleaccording to an embodiment of the invention;

FIG. 2G illustrates schematically a pillar of a pillared particleaccording to an embodiment of the invention;

FIG. 3A illustrates schematically the formation of a pillared particleby an etching process according to an embodiment of the invention;

FIG. 3B illustrates schematically the formation of a pillared particleby a growth process according to an embodiment of the invention;

FIG. 4A illustrates schematically a pillared particle according to anembodiment of the invention formed by an etching process and havingpillars of a first average length;

FIG. 4B illustrates schematically a pillared particle according to anembodiment of the invention formed by an etching process and havingpillars of a first average length;

FIG. 5A illustrates schematically a pillared particle according to anembodiment of the invention;

FIG. 5B illustrates schematically a pillared particle according to anembodiment of the invention;

FIG. 5C illustrates schematically a pillared particle according to anembodiment of the invention;

FIG. 5D illustrates schematically a pillared particle according to anembodiment of the invention;

FIG. 5E illustrates schematically a pillared particle according to anembodiment of the invention;

FIG. 6 is a graph of size distribution of a starting material powderaccording to an embodiment of the invention;

FIG. 7 is a first SEM image of a powder according to an embodiment ofthe invention;

FIG. 8 is a second SEM photograph of the powder of FIG. 7;

FIG. 9 is a graph of size distribution of a pillared particle powderaccording to an embodiment of the invention;

FIG. 10 is a plot of discharge capacity vs number of discharge cyclesfor two lithium ion cells containing relatively small particles and alithium ion cell containing relatively large particles;

FIG. 11 is a SEM image of pillars of a pillared particle;

FIG. 12 is a plot of cell voltage against discharge capacity/chargecapacity for a lithium ion cell containing relatively small pillaredparticles at a range of discharge rates;

FIG. 13 is a plot of cell voltage against discharge capacity/chargecapacity for a lithium ion cell containing relatively large pillaredparticles at a range of discharge rates;

FIG. 14 is a SEM image of pillars of a small pillared particle;

FIG. 15 is a plot of charge capacity against cell electrode thicknesschange for lithium ion cells containing a range of pillared particlesizes and for cells containing particles that do not carry pillars;

FIG. 16A is a SEM image of a first pillared particle having a highaspect ratio core;

FIG. 16B is a SEM image of a second pillared particle having a highaspect ratio core;

FIG. 17 is a SEM image of a powder containing etched silicon that doesnot carry pillars;

FIG. 18 is a plot of BET vs average first cycle loss for etched siliconpillared particles; and

FIG. 19 is a SEM image of a powder containing pillared siliconparticles.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described herein with reference to lithium ionbatteries and insertion and desorption of lithium ions, however it willbe appreciated that the invention may be applicable to other metal ionbatteries, for example sodium, potassium or magnesium ion batteries.

Pillared Particle Structure

“Pillared particles” as used herein mean particles comprising a particlecore and a plurality of spaced-apart pillars extending therefrom. It isalso to be understood that the pillar may be a wire, nanowire, rod,column, filament, thread, tube, cone or any other elongated structureextending from a particle core.

The pillared particles comprise an electroactive material such asgraphite, graphene, hard carbon, silicon, germanium, gallium, tin,aluminium, lead, indium, antimony, bismuth, oxides, nitrides or hydridesthereof, mixtures of these, mixtures or composite alloys containingthese elements and chalcogenides and ceramics that are electrochemicallyactive. One exemplary active material is silicon which can insert andrelease lithium ions. The insertion of lithium ions into silicon oranother electroactive material can be described as lithiation and theremoval of the lithium can be described as delithiation. At least someof the plurality of pillars in a pillared particle comprise anelectroactive material. The composition of the core and the pillars maybe identical or different. Where the pillars and the core are ofdifferent compositions, both the pillars and the core may comprise anelectroactive material. Alternatively only the pillars may comprise anelectroactive material. Where only the pillars comprise an electroactivematerial and the core is formed from a non-electroactive material, thecore preferably comprises an electronically conductive material.

The active material may be a material that undergoes expansion duringinsertion of metal ions. The expansion may be due to structural changesof the anode caused by formation of an alloy of the active material andthe metal ions, for example a Si—Li alloy formed by insertion of lithiumions by silicon. Tin is another example of an active material thatexpands on metal ion insertion. The volume of an active material uponmetallation, e.g. lithiation, to its maximum capacity may be at least10% larger than its volume when substantially unmetallated. Exemplarymaterials that undergo an expansion of at least 10% include silicon andtin. The volume change of an active material upon metallation to itsmaximum capacity may be determined by computer modelling.

The core may be a single doped or undoped material, for example p-doped,n-doped or undoped silicon, or may comprise more than one material. Forexample, the core may comprise a first material at the core centre,which may or may not be electroactive, coated with an electroactiveshell formed from a different second material. For example, the core maycomprise a carbon centre coated with a silicon shell. The coating mayprovide a core surface that partially or fully covers the material atthe core centre. In the case where the core material is carbon,exemplary cores include hard carbon, graphite and graphene.

The pillars may be the same material as or a different material to thematerial forming the core or core surface. In the case where the pillarsand core are the same material, the pillars may be integral with thecore surface. The plurality of pillars can be formed or fabricated on orattached to one or more surfaces of the particle core in a regular orirregular, ordered or disordered array or in a random scattereddistribution.

With reference to FIG. 2A, pillars 205 may be attached at one end to asurface of the core 207 and extend outwards substantially perpendicularto that surface, or may extend at an angle θ that is substantially lessthan 90 degrees as illustrated in FIG. 2B. Preferably the angle θ is15-90 degrees, more preferably 40-90 degrees.

Each pillar may carry one or more branches branching from the pillar, asshown in FIG. 2C.

The pillars may include one or more kinks or changes in direction, asshown in FIG. 2D.

A plurality of pillars may carry a lintel 209, as shown in FIG. 2E. Thislintel may be a porous structure that remains as an artefact of astarting material that has been etched to form a pillared particle, asdescribed in more detail below. The pillars may be solid or porous ormay comprise a solid core with a porous outer surface. The surface ofthe pillars may be smooth or rough.

The pillars may have cross sections that are substantially circular ormay form other substantially regular or irregular shapes. Examples ofregular shaped cross-sections include squares, rectangles, diamonds, orstars or variations of such shapes where the sides of the pillars haveconvex or concave surfaces rather than straight sides. Irregularcross-sectional shapes may for example include shapes formed from acombination of the aforementioned substantially regular cross-sectionalsshapes.

The ends of the pillars may be spaced apart from the core surface.

The ends of one or more pillars may be joined together. This joining ofends may be an artefact of a method of forming a pillared particlewherein the pillars have been formed in a solution and/or the pillarsare washed after manufacture and dried such that capillary action andsurface tension causes adjacent pillars to be adhered to each other.

The width of the pillars may be substantially constant along at leastpart of the length of the pillars, or the width of the pillars may varyalong their length. For example, the pillar may be a tapered structurehaving a width W1 at its base that is larger than a width W2 away fromthe base, as illustrated in FIG. 2F.

The pillars are spaced apart on the particle. In operation in the anodeof a lithium ion battery (i.e. during charging and/or discharging of thebattery), lithium ions are inserted into the electroactive pillars ofthe pillared particles during charging (also referred to as lithiation)and are released during discharge of the battery (also referred to asdelithiation). During charging there is a significant expansion in thevolume of the electroactive pillars due to the incorporation of lithiumions and during discharge there is a corresponding contraction of thepillar volume from delithiation. It has been observed that the volumeexpansion of the electroactive pillars during charge is substantially inthe radial to lateral direction, for example it results in a pillar ofincreased diameter whilst the height is relatively unchanged orundergoes a relatively small change. Spacing apart of the pillarsprovides space into which the electroactive pillars may expand andcontract without impeding each other, which reduces mechanical stressexperienced by the pillars, that could otherwise lead to cracking,detachment and/or disintegration of the pillars from repeated insertionand desorption of lithium. The amount of radial expansion of the pillarsinto the spaces between them during charging may depend on the type ofelectroactive material contained in the pillars, the maximum amount ofmetal ions inserted into the pillars, the porosity of the pillars, theirshape and other factors.

Preferably, the thickness of a composite electrode layer (excluding anysubstrate or current collector) containing pillared particles asdescribed herein expands by less than 150%, preferably less than 125%,when charged for the first time (i.e. with no pre-lithiation) to 3,000mAh/g, the capacity being per gram of silicon in the anode.

If other active materials are present in the composite electrode, forexample active carbon, then capacity may be per gram of active material.Preferably, at least 5 weight %, at least 10 weight %, at least 20weight % or at least 50 weight % of the active material is siliconactive material (either in the form of a material consisting essentiallyof silicon or as a composite material having silicon at a surfacethereof).

“Composite electrode” as used herein means a composition of at least oneactive material and one or more further materials. The one or morefurther materials may be selected from, without limitation, binders,further active materials such as active carbon, and non-activeconductive materials, for example carbon black. The composite electrodedoes not include cell components that the composite electrode may be incontact with when in use, such as a current collector or electrolyte.The composite electrode is a solid composition. The constituents of thesolid composite electrode may be dispersed in one or more solvents toform a slurry that may be deposited on a surface, in particular acurrent collector layer, followed by evaporation of the one or moresolvents to form a composite electrode layer.

Optionally, pillared particles make up at least 5 weight %, at least 10weight %, at least 20 weight %, at least 50 weight % or at least 60weight % of a composite electrode.

Preferably electrode thickness expansion upon charging to 2,000 mAh/g isless than 60%, more preferably less than 50%.

Preferably electrode thickness expansion upon charging to 1,500 mAh/g isless than 35%, more preferably less than 30%.

The change in thickness of an electrode in an electrochemical cell maybe measured as the cell is charged (first cycle) with an El-Cell®Electrochemical Dilatometer ECD-nano placed inside a temperaturecontrolled chamber at 20° C.

Furthermore the plurality of spaced pillars increases the surface areaof electroactive material in the pillared particle that can be contactedwith the electrolyte in the battery. This increases the rate at whichthe lithium (or other metal ion) can be inserted into the electroactivematerial and aids the uniform insertion density of metal ions throughoutthe active material. Additionally, in a cell with liquid electrolyte, byproviding enough spacing between pillars so that when they are fullyexpanded, space remains around them such that the electrolyte can remainin contact with the pillar and core surface without being squeezed out,then lithium loss during cycling can be reduced. For example, if thereis not enough space between the pillars to accommodate the fullexpansion of the pillars during charge then the liquid electrolyte willbe forced away from the particle surface and no longer be in contactwith the surface of the pillars or core. In this case, during dischargeit may be more difficult for all the lithium to be released and somecould remain trapped in the pillars and/or particle core. Also, if therate of release of the metal ions varies throughout the particle, peakmechanical stresses on contraction could increase, leading to fractureof the electroactive material.

In one arrangement, substantially all of the pillars are spaced apartfrom one other. In another arrangement, the pillared particle maycomprise at least some clusters of pillars as illustrated in FIG. 2G.The pillared particle may comprise both clusters of pillars and pillarsthat are spaced apart. The spacing between pillars and/or clusters ofpillars may be regular or irregular. Preferably, the average distancebetween a pillar or pillar cluster and its adjacent pillars or pillarclusters is at least half the width of the pillar or pillar cluster.More preferably, the average distance between adjacent pillars or pillarclusters is at least the width of the pillar or pillar cluster. Thewidth of a pillar is the pillar diameter in the case of a substantiallycylindrical pillar.

In one preferred arrangement, at least some of the pillars of a pillaredparticle are substantially perpendicular to one or more surfaces of theparticle core; are unbranched and are substantially straight.

An average pillar density of the pillars on the particle core may be inthe range of about 0.1-80%, optionally 10-80%. These ranges may providea balance between a maximum number of electroactive pillars availablefor lithium insertion and a reduced number of pillars to avoid crackingof the pillared particles and to provide space to avoid electrolytebeing forced away from the particle surfaces.

Coverage can be defined by an average pillar density given by theformula A/(A+B)×100% where A is the area of a surface of the particlecore occupied by pillars and B is the area of the same surface that isunoccupied by pillars. The average pillar density can be calculated fora single surface, several surfaces or for all surfaces of the particlecore. Generally, it should be understood that unless otherwise stated,average pillar densities cited herein are calculated using the areas ofsurfaces occupied by pillars and individual surfaces of the particlecore which do not contain any pillars are not included in thecalculation.

To achieve an appropriate mass of electroactive pillars in a pillaredparticle, the average pillar density may be at least 0.1%, preferably atleast 1%, more preferably at least 5% and most preferably at least 10%.For reasons given earlier it may be disadvantageous if the averagepillar density is too high, preferably it is no more than 80%, morepreferably it is no more than 60% and most preferably it is no more than50%.

The pillars may have a length in the range 0.2 or 1 microns up to about4 microns, optionally up to about 2 microns. The pillar length ispreferably less than 10 microns.

The mean average thickness of the pillars may be at least 10 nm,optionally at least 20 nm and may be less than 1 μm. The mean averagethickness may be a pillar diameter in the case of pillars with asubstantially circular cross-section. In the case where the pillaredparticles include pillars with substantially non-circular or irregularcross-sectional forms, it will be appreciated that the mean averagepillar thickness relates to the smallest dimension of thecross-sectional shape.

The mean average pillar thickness may be in the range of about 10-250nm, optionally about 30-150 nm. The pillars may have a mean averagepillar thickness of less than 80 nm. In the case where the pillaredparticles include pillars that are clustered together, it will beappreciated that the mean average pillar thickness relates to thethickness of the individual pillars, and not to thicknesses of pillarclusters Elongated structures or pillars with these diameters areideally suited to withstand the expansion and contraction during chargeand discharge without cracking, fracturing or disintegration. If thediameter becomes too small, for example less than 10 nm, then the highsurface area to volume ratio of the pillars contributes to anexcessively high lithium loss during operation of a cell from formationof a Surface Electrolyte Interphase (SEI) layer on the surface of thesilicon and reduces the lifetime of a cell.

The pillared particles may have at least one first dimension (asmeasured along a single direction across the pillared particle includingthe core and pillars in the size measurement) of less than 10 μm.Another dimension of the pillared particle, which may be orthogonal tothe first dimension, can be longer but is preferably no more than 50 μmand is preferably no more than 25 μm, most preferably no more than 20μm.

The dimensions of the pillared particle, including length and thicknessof pillars, may be measured by scanning electron microscopy ortransmission electron microscopy. Mean average lengths and thicknessesmay be obtained by measuring lengths and thicknesses for a plurality ofpillars in a sample of a pillared particle material.

A composition or powder comprising a plurality of pillared particles isused in forming the anode of a lithium ion battery. The plurality ofpillared particles may have a size distribution. Substantially all ofthe pillared particles in the composition may have at least onedimension of 10 μm or less. Alternatively, the composition may includepillared particles that do not have at least one dimension of 10 μm orless.

A distribution of the particle sizes of a powder of starting materialparticles used to form pillared particles may be measured by laserdiffraction, in which the particles being measured are typically assumedto be spherical, and in which particle size is expressed as a sphericalequivalent volume diameter, for example using the Mastersizer™ particlesize analyzer available from Malvern Instruments Ltd. A sphericalequivalent volume diameter is the diameter of a sphere with the samevolume as that of the particle being measured. If all particles in thepowder being measured have the same density then the sphericalequivalent volume diameter is equal to the spherical equivalent massdiameter which is the diameter of a sphere that has the same mass as themass of the particle being measured. For measurement the powder istypically dispersed in a medium with a refractive index that isdifferent to the refractive index of the powder material. A suitabledispersant for powders of the present invention is water. For a powderwith different size dimensions such a particle size analyser provides aspherical equivalent volume diameter distribution curve.

Size distribution of particles in a powder measured in this way may beexpressed as a diameter value Dn in which at least n % of the volume ofthe powder is formed from particles have a measured spherical equivalentvolume diameter equal to or less than D.

Preferred size distributions for a powder of starting material particlesinclude one or more of the following:

D10≤10 μm

D50≤25 μm, optionally ≤15 μm, optionally ≤10 μm

D90≤25 μm, optionally ≤15 μm

D10≥0.5 μm, optionally ≥1 μm

If a pillared particle is formed by etching a starting materialparticle, for example as described with reference to FIG. 3A below, orby growing pillars out of the starting material particle, then it willbe appreciated that the particle core of the resultant pillared particlewill be smaller than the starting material particle.

If the pillared particles are formed by growing or attaching pillarsonto to the surface of a starting material particle, for example asdescribed with reference to FIG. 3B, then it will be appreciated thatthe particle core of the resultant pillared particle will besubstantially the same size as the starting material particle.

Therefore, if a starting material powder has a D10 value of ≤10 μm thenit will be appreciated that the particle core of pillared particles in aproduct powder formed using this starting material powder must also havea D10 value of ≤10 μm, regardless of whether the pillared particles areformed by etching the particles of a starting material powder or bygrowth or attachment of pillars to the particles of a starting materialpowder.

As an alternative to using the size distribution of the startingmaterial to determine a maximum possible size distribution of theproduct, Dn size distribution values of pillared particles may bemeasured directly. The Dn values of a pillared particle may relate to adiameter of a sphere having a surface that encompasses the core and thepillars in the case of a pillared particle with rigid pillars, forexample pillars formed by etching silicon of a starting material, or mayrelate substantially to a diameter of a sphere having a surface thatencompasses the core only in the case of a pillared particle withflexible pillars. Preferred size distributions for pillared particleproducts are as described above for starting materials.

An example measurement system for measuring the shapes and dimensions ofparticles in a powder of pillared particles or a powder of startingmaterial particles using an optical microscope or SEM with digital imageprocessing is Morphologi™, also available from Malvern Instruments Ltd.In this technique a 2D projection of the area of each particle iscaptured and the particle dimensions and shape can be measured andclassified.

Pillared particles having at least one dimension of less than 10 μm maybe more easily dispersed and incorporated into composite layers for highcapacity anodes for reasons described herein. Additionally, if theparticle core comprises an electroactive material which undergoes alarge volume expansion and contraction during operation, a smaller coresize may enable the particle core to insert and release more lithium (orother metal ion) without cracking or fracture of the core that may occurif larger pillared particles are used. A battery using these pillaredparticles as an active material may be charged to a higher capacity perunit mass or per unit volume than a battery comprising larger pillaredparticles, with little or no loss of stability.

Pillared particles having at least one dimension of less than 10 μm or apowder of pillared particles where the D10 value of the particle coresis less than 10 μm may also enable the formation of an anode layer thatis thinner than an anode formed from pillared particles that do not haveat least one dimension of less than 10 μm.

The inventors have found that it is easier to prepare thin compositeanode coatings, for example a coating with an average thickness lessthan 60 μm, with a uniform thickness and homogeneously dispersedcomponents using pillared particles of this size. Thin anode coatings(or layers) may be required to balance the cathode in a cell whichtypically has a much lower volumetric charge capacity than an anodecomprising an electroactive material such as silicon. The thickness maybe measured by observing cross sections of the anode coating producedusing a microtome. The average thickness may also be calculated bymeasuring the mass of the anode coating per unit area if the densitiesand mass ratios of the components in the anode coating are knowntogether with the coating porosity.

If pillars are formed by growth of nanowires on a starting material, asdescribed in more detail below, then the nanowire core and pillars mayhave the dimensions described above, however nanowire pillars may have amean length of no more than 10 times the mean average size of the core.

Surface area per unit mass of the pillared particle may be measured byvarious techniques including BET and laser diffractometry. The specificsurface area measured using the BET (Brunauer, Emmett and Teller)technique may be less than 200 m²/g. Preferably it is less than 100m²/g, more preferably it is less than 60 m²/g or less than 50 m²/g, mostpreferably it is less than 35 m²/g. The specific surface area measuredusing the BET technique may be more than 0.1 m²/g, preferably it is morethan 1 m²/g and more preferably it is more than 5 m²/g. A higherspecific surface area promotes the interaction of the metal ions withthe active material, aiding a uniform insertion density of metal ionsthroughout the active material and enabling faster charge/dischargerates. However, if the specific surface area is too large then thecharge capacity per unit mass and/or cycle life may be reduced throughexcessive formation of oxide and/or SEI layer on the surface of theactive material. The specific surface area may be dependent on, forexample, the size and density of the pillars, the porosity or surfaceroughness of the pillars and the size of the particle core.

Preferably the plurality of pillared particles in a powder used to forma composite are substantially discrete from one another. A “discretepillared particle” as described herein means a pillared particle that isnot joined or bound to another pillared particle. In a composite anodecomprising a plurality of pillared particles, preferably duringcharging/discharging the relative movement from expansion andcontraction of the electroactive material of each pillared particle issubstantially independent of the movement from expansion and contractionof other nearby pillared particles. Preferably, the pillars of differentpillared particles are not substantially intertwined or entangled.Pillared particles with pillars having preferred dimensions describedabove may avoid intertwining due to their short length, and due to thepillars being relatively inflexible as a result of their short length.Use of a composition containing pillared particles that remainsubstantially discrete from one another and/or experience relativemovement during charging/discharging substantially independent of eachother may reduce or eliminate the phenomenon of “lift” or “heave”resulting from expansion of an anode formed from a single block orinterconnected mass of active material. Moreover, use of discreteparticles in an anode may provide good contact between the pillaredparticles and the electrolyte. It may be more difficult for theelectrolyte to wet the surfaces of active pillars in a tangled mass. Itmay also be more difficult to disperse the active particles uniformlywithin an electrode slurry or composite if the pillared particles arenot substantially discrete or become entangled due to clumping of theentangled particles. It will be understood that the discrete pillaredparticles of a powder or composition may contain discrete pillaredparticles that may come into physical contact with each other and/orwith other components, for example a binder or electrolyte, and that thediscrete pillared particles may be contained within a matrix defined bya binder or other matrix material.

The pillared particles may be joined to each other after formation of acoating or composite, for example, sintering of a layer of pillaredparticles may be performed to provide a self supporting sinteredcomposite.

Pillar Mass Fraction and Pillar Volume Fraction

The Pillar Mass Fraction (PMF) of a pillared particle is provided by thefollowing equation:PMF=[(Mass of pillars attached to and extending from the particlecore)/(Total mass of pillared particle)]×100%

Accordingly, in the case of a silicon active pillared particle materialit will be understood that the PMF is the mass of silicon pillarsdivided by the mass of the whole particle.

The PMF may be determined by various methods. If the pillars are grownon, deposited on or attached to the particle cores then the PMF may becalculated by measuring the mass of a plurality of particle cores beforegrowth or attachment and the mass of the pillared particles after growthor attachment and subtracting one from the other to calculate the massof pillars in the above equation.

If the pillared particle is made by etching a silicon particle to formsilicon pillars on the surface of a particle core then the PMF may bedetermined by an oxidation technique. This involves firstly measuringthe mass of a quantity of pillared particles and then measuring a changein mass over time of the quantity of pillared particles duringoxidation, for example by heating pillared particles in anoxygen-containing atmosphere. e.g. by heating to 1040° C. in air. Thepillars are fully oxidised first, and oxidise at a relatively rapid rate(shown as a relatively rapid increase in the rate of mass increase).Oxidation of the pillars is deemed to be complete when the rate of massincrease is observed to reduce and become linear with time. From thistime onwards the rate of mass increase is due only by steady oxidationof the silicon into the particle core. The observed increase in mass upto this point is mostly due to oxidation of the pillars and using thedifference in density between silicon and silicon oxide, the mass of thepillars before oxidation and hence the PMF can be determined. For apowder sample with a broad size distribution, the particles cores of thesmaller pillared particles may additionally be oxidised and a correctionfactor may need to be applied to take account of the core oxidation. Thecorrection factor can be estimated by doing the measurement on a samplecomprising the particle cores with the pillars absent or removed. Thismethod is particularly suitable for pillared particles having siliconpillars.

The PMF may also be determined by measuring the mass of a quantity ofpillared particles, removing the pillars from the particle cores, forexample by mechanical agitation (such as ultrasonication), scraping orchemical etching, separating the detached pillars from the particlecores and measuring either the mass of the quantity of particle coresand/or the mass of the detached pillars. This method is preferredbecause it may be applied to pillared particles of any material.

The PMF may be affected by, for example, the average length of pillars,their porosity and the percentage coverage of the particle core by thepillars (the pillar density). The PMF is preferably greater than orequal to 5%, more preferably at least 10%, most preferably at least 20%.The PMF is preferably no more than 95%, more preferably no more than80%. Most preferably the PMF is 20-60%, especially 25-50%. A higher PMFvalue means that the high capacity active pillars make a largercontribution to the active mass of the electrode and a higher overallcapacity per unit mass can be obtained. However, if the PMF value is toohigh then the cost of manufacturing the pillared particles may increaseso that the cost to performance ratio of the electrode materials becomesuncompetitive, the pillars may become too densely packed and/or themechanical/electronic integrity of the pillar to core connection may beweakened.

If the material of the particle core has a density significantlydifferent from the density of the material forming the pillars, then thePillar Volume Fraction (PVF) may be measured instead of PMF, although itwill be appreciated that PVF is applicable to the cases in which thecore and pillar densities are substantially the same (in which case thePVF value will be substantially the same as the PMF value) and the casein which the core and pillar densities are significantly different. ThePVF is given by the following equation:PVF=[(Total volume of pillars extending from the particle core)/(Totalvolume of pillared particle)]×100%

Similar methods to those used for measuring PMF may be used to measurePVF. Moreover, PVF may be derived from PMF measurements using a ratio ofdensities of the core material and the pillar material. The volumes ofthe pillars and the pillared particles are the volumes which do notinclude volumes of open pores. Closed pores or voids that are completelyenclosed within the bulk of a core or pillar are included in thevolumes. Accordingly, if the pillars or cores are porous, the porositymay need to be measured. Example techniques that may be used to measureporosity include mercury porosimetry and Barret-Joyner-Halenda (BJH)analysis.

Volumes of pillars and of pillared particles may be measured using aMasterSizer system or other similar laser diffractometry device, asdescribed above. In an exemplary process, the volume of a pillaredparticle is measured; pillars are detached from the pillared particlesby a mechanical process such as ultrasonication; and the volume of thepillars is measured. In the case of porous pillars or cores, theporosity is determined and the measured volume is adjusted. For example,if porosity is 5% then measured volume is adjusted by 0.95 to give asolid volume. The volumes may also be measured using 2D digital imagingsystems such as Morphologi, as described above, though they typicallyare unable to resolve particles with a dimension below 0.5 μm.

The PVF may be affected by, for example, the average length of pillarsand the percentage coverage of the particle core by the pillars (thepillar density) and the density of the particle core and pillarmaterials. The PVF is preferably greater than or equal to 5%, morepreferably at least 10%, most preferably at least 20%. The PVF ispreferably no more than 95%, more preferably no more than 80%. Mostpreferably the PVF is 20-60%, especially 25-50%. A higher PVF valuemeans that the high capacity active pillars make a larger contributionto the active mass of the electrode and a higher overall capacity perunit volume can be obtained. However, if the PVF value is too high thenthe cost of manufacturing the pillared particles may increase so thatthe cost to performance ratio of the electrode materials becomesuncompetitive, the pillars may become too densely packed and/or themechanical/electronic integrity of the pillar to core connection may beweakened.

Preferably the BET/PMF ratio of a powder of the pillared particles ispreferably less than 3, less than 2, less than 1.5 or less than 1,wherein BET is the specific surface area of the pillared particles inm²/g and PMF is expressed a percentage as per the above equation.

Preferably, the BET/PMF ratio is greater than 0.1.

It will be understood that the BET/PMF ratio is an average value forpillared particles in a pillared particle powder.

Although an increase in PMF may increase BET, the relationship betweenPMF and BET is not linear (and it can for example be affected by thesurface roughness or porosity of the pillars and core). The presentinventors have found that the above BET/PMF ratio may exclude materialsin which one of PMF and BET is too high or too low, leading to thedisadvantages of a PMF or BET value that too low or that is too high, asdescribed above.

Specific Charge Capacity of the Pillared Particles

The pillared particles preferably have a specific reversible chargecapacity of at least 500 mAh per gram of pillared particle mass. Thereversible charge capacity is the charge provided by discharge of thepillared particles in the anode of the cell after a full charge cycle.More preferably the pillared particles have a reversible charge capacityof at least 800 mAh/g, most preferably at least 1,000 mAh/g andespecially at least 1,800 mAh/g. Preferably these reversible chargecapacities are sustained for at least 50 charge/discharge cycles, morepreferably at least 100 charge/discharge cycles, most preferably atleast 200 charge/discharge cycles and especially at least 300charge/discharge cycles.

Starting Materials for the Particle Core

The starting material for the particle core is preferably in particulateform, for example a powder, and the particles of the starting materialmay have any shape. For example, the starting material particles may becuboid, cuboidal, substantially spherical or spheroid or flake-like inshape. The particle surfaces may be smooth, rough or angular and theparticles may be multi-faceted or have a single continuously curvedsurface. The particles may be porous or non-porous.

A cuboid, multifaceted, flake-like, substantially spherical or spheroidstarting material may be obtained by grinding a precursor material, forexample doped or undoped silicon as described below, and then sieving orclassifying the ground precursor material. Exemplary grinding methodsinclude power grinding, jet milling or ball milling. Depending on thesize, shape and form of the precursor material, different millingprocesses can produce particles of different size, shape and surfacesmoothness. Flake-like particles may also be made by breakingup/grinding flat sheets of the precursor material. The startingmaterials may alternatively be made by various deposition, thermalplasma or laser ablation techniques by depositing a film or particulatelayer onto a substrate and by removing the film or particulate layerfrom the substrate and grinding it into smaller particles as necessary.

Samples or powders of the starting material particles may have D90, D50and/or D10 values as described above.

In the case where a pillared particle is formed by etching a granularstarting material having at least one dimension of less than 10 microns,it will be appreciated that at least one dimension of the pillaredparticles produced will likewise be no more than 10 microns. Dependingon the degree and type of etching, one or more dimensions of thepillared particle may be less than the corresponding dimension of thestarting material. In the case where a pillared particle is formed byetching, the starting material comprises an electroactive material asdescribed above. Preferably it comprises an electroactive material thatundergoes a volume expansion of at least 10% upon complete insertion bythe material of the metal ion of a metal ion battery.

The starting material may comprise particles of substantially the samesize. Alternatively, the starting material may have a distribution ofparticle sizes. In either case, sieves and/or classifiers may be used toremove some or all starting materials having maximum or minimum sizesoutside desired size limits.

In the case where the pillared particle is formed by etching a materialcomprising silicon, the starting material may be undoped silicon ordoped silicon of either the p- or n-type or a mixture, such as silicondoped with germanium, phosphorous, aluminium, silver, boron and/or zinc.It is preferred that the silicon has some doping since it improves theconductivity of the silicon during the etching process as compared toundoped silicon. The starting material is optionally p-doped siliconhaving 10¹⁹ to 10²⁰ carriers/cc.

Silicon granules used to form the pillared particles may have asilicon-purity of 90.00% or over by mass, for example 95.0% to 99.99%,optionally 98% to 99.98%.

The starting material may be relatively high purity silicon wafers usedin the semiconductor industry formed into granules. Alternatively, thegranules may be relatively low purity metallurgical grade silicon, whichis available commercially and which may have a silicon purity of atleast 98%; metallurgical grade silicon is particularly suitable becauseof the relatively low cost and the relatively high density of defects(compared to silicon wafers used in the semiconductor industry). Thisleads to a low resistance and hence high conductivity, which isadvantageous when the pillar particles or fibres are used as anodematerial in rechargeable cells. Impurities present in metallurgicalgrade silicon may include Iron, Aluminium, Nickel, Boron, Calcium,Copper, Titanium, and Vanadium, oxygen, carbon, manganese andphosphorus. Certain impurities such as Al, C, Cu, P and B can furtherimprove the conductivity of the starting material by providing dopingelements. Such silicon may be ground and graded as discussed above. Anexample of such silicon is “Silgrain™” from Elkem of Norway, which canbe ground and sieved (if necessary) to produce silicon granules, thatmay be cuboidal and/or spheroidal.

The granules used for etching may be crystalline, for example mono- orpoly-crystalline with a crystallite size equal to or greater than therequired pillar height. The polycrystalline granules may comprise anynumber of crystals, for example two or more.

Where the pillared particles are made by a growth technique as describedbelow, the starting material may comprise an electroactive material asdescribed above. The starting material in this case may also comprisemetal or carbon based particles. Carbon based starting materials maycomprise soft carbon, hard carbon, natural and synthetic graphite,graphite oxide, fluorinated graphite, fluorine-intercalated graphite,graphene, carbon nanotubes (CNT), carbon fibres and multi-walled carbonnanotubes (MWCNT).

Graphene based starting materials may comprise particles comprising aplurality of graphene nanosheets (GNS) and/or oxidised graphenenanosheets (ox-GNS) or nano Graphene Platelets (NGP). Methods of makinggraphene particles include exfoliation techniques (physical, chemical ormechanical), unzipping of MWCNT or CNT, epitaxial growth by CVD and thereduction of sugars. Graphene based particles used as starting materialsfor the core of pillared particles preferably have an initial reversiblecharge capacity (on the first full charge cycle) of at least 400 mAh pergram of graphene particle, more preferably at least 500 mAh/g, mostpreferably at least 800 mAh/g and especially at least 1,000 mAh/g.

Methods of Pillared Particle Formation

FIG. 3A illustrates a first method of forming pillared particles whereina starting material is etched to form a pillared particle wherein astarting material 301 is exposed to an etching formulation for selectiveetching at the surface of the starting material to produce a pillaredparticle 303 having a core 305 and pillars 307.

It will be appreciated that the volume of the particle core of thepillared particle formed by this method is smaller than the volume ofthe starting material, and the surface of the core is integral with thepillars. The size of the pillared particle may be the same as or lessthan the size of the starting material.

A suitable process for etching a material having silicon at its surfaceis metal-assisted chemical etching (alternatively called galvanicexchange etching or galvanic etching) which comprises treatment of thestarting material with hydrogen fluoride, a source of silver ions whichelectrolessly deposit onto the surface of the silicon and an oxidant,for example a source of nitrate ions. More detail on suitable etchingprocesses can be found in, for example, Huang et al., Adv. Mater. 23, pp285-308 (2011).

The etching process may comprise two steps, including a nucleation stepin which silver nanoclusters are formed on the silicon surface of thestarting material and an etching step. The presence of an ion that maybe reduced is required for the etching step. Exemplary cations suitablefor this purpose include nitrates of silver, iron (III), alkali metalsand ammonium. The formation of pillars is thought to be as a result ofetching selectively taking place in the areas underlying the silvernanoclusters. It is also known that metal-assisted etching of siliconcan produce pillars with porous walls (for example as described in C.Chartier et al., Electrochimica Acta 2008, 53, p 5509), the level ofporosity being dependent on dopant levels and the ratios of thecomponents in the etching solution.

The nucleation and etching steps may take place in a single solution ormay take place in two separate solutions.

Silver may be recovered from the reaction mixture for re-use.

Exemplary etching processes suitable for forming pillared particles aredisclosed in WO 2009/010758 and in WO 2010/040985.

Other etching processes that may be employed include reactive ionetching, and other chemical or electrochemical etching techniques,optionally using lithography to define the pillar array.

If the pillared particle comprises a first material at its core centrewith a shell formed from a second material, for example carbon coatedwith silicon as described above, then this particle may be formed byetching of silicon-coated carbon to a depth of less than the thicknessof the silicon shell in order to form a pillared particle with acomposite carbon/silicon core.

The pillars may also be formed on or attached to a particle core usingmethods such as growing, adhering or fusing pillars onto a core orgrowing pillars out of a core. FIG. 3B illustrates a second method offorming pillared particles wherein pillars 307, for example nanowires,are grown on or attached to a starting material 301 such as a silicon orcarbon (e.g. graphite or graphene) starting material. The volume of theparticle core 305 of the resultant pillared particle 303 may besubstantially the same as the volume of the starting material 301. Inother words, the surface of the starting material may provide thesurface of the particle core 305 from which the pillars 307 extend.

Exemplary methods for growing pillars include chemical vapour deposition(CVD) and fluidised bed reactors utilising the vapour-liquid-solid (VLS)method. The VLS method comprises the steps of forming a liquid alloydroplet on the starting material surface where a wire is to be grownfollowed by introduction in vapour form of the substance to form apillar, which diffuses into the liquid. Supersaturation and nucleationat the liquid/solid interface leads to axial crystal growth. Thecatalyst material used to form the liquid alloy droplet may for exampleinclude Au, Ni or Sn.

Nanowires may be grown on one or more surfaces of a starting material.

Pillars may also be produced on the surface of the starting materialusing thermal plasma or laser ablation techniques.

The pillars may also be formed by nanowire growth out of the startingmaterial using methods such as a solid-liquid-solid growth technique. Inone example silicon or silicon-based starting material granules arecoated with catalyst particles (e.g. Ni) and heated so that a liquidalloy droplet forms on the surface whilst a vapour is introducedcontaining another element. The vapour induces condensation of a productcontaining the starting material and the other element from the vapour,producing growth of a nanowire out of the starting material. The processis stopped before all of the starting material is subsumed intonanowires to produce a pillared particle. In this method the core of thepillared particle will be smaller than the starting material.

Silicon pillars grown on or out of starting materials may be grown asundoped silicon or they may be doped by introducing a dopant during thenanowire growth or during a post-growth processing step.

Particle Core

Particle cores illustrated in FIGS. 3 and 4 are substantially spherical,however the particle core may have any shape, including substantiallyspherical, spheroidal (oblate and prolate), and irregular or regularmultifaceted shapes (including substantially cube and cuboidal shapes).The particle core surfaces from which the pillars extend may be smooth,rough or angular and may be multi-faceted or have a single continuouslycurved surface. The particle core may be porous or non-porous. Acuboidal core may be in the form of a flake, having a thickness that issubstantially smaller than its length or width such that the core hasonly two major surfaces.

The aspect ratio of a pillared particle core having dimensions of lengthL, width W and thickness T is a ratio of the length L to thickness T(L:T) or width W to thickness T (W:T) of the core, wherein the thicknessT is taken to be the smallest of the 3 dimensions of the particle core.The aspect ratio is 1:1 in the case of a perfectly spherical core.Prolate or oblate spheroid, cuboidal or irregular shaped corespreferably have an aspect ratio of at least 1.2:1, more preferably atleast 1.5:1 and most preferably at least 2:1. Flake like cores may havean aspect ratio of at least 3:1.

In the case of a substantially spherical core, pillars may be providedon one or both hemispheres of the core. In the case of a multifacetedcore, pillars may be provided on one or more (including all) surfaces ofthe core. For example, in the case of a flake core the pillars may beprovided on only one of the major surfaces of the flake or on both majorsurfaces.

The core material may be selected to be a relatively high conductivitymaterial, for example a material with higher conductivity than thepillars, and at least one surface of the core material may remainuncovered with pillars. The at least one exposed surface of theconductive core material may provide higher conductivity of a compositeanode layer comprising the pillared particles as compared to a particlein which all surfaces are covered with pillars.

FIG. 5A illustrates an embodiment in which the core 505 is formed from arelatively high conductivity material, for example a graphite particle,graphene sheet or a graphene particle comprising more than one graphenesheet, and silicon nanowires 507 are grown on one surface of the core.Alternatively the core can comprise a doped silicon material. The aspectratio, that is the ratio of length L to thickness T, is greater than 3:1in this example.

FIG. 5B illustrates an embodiment in which pillars are provided onopposing surfaces of a core such as a graphene core or silicon flake.

FIG. 5C illustrates an embodiment in which the core is an oblatespheroid.

FIG. 5D illustrates an embodiment in which the core is multifaceted andhas an irregular shape. Pillars are provided on some facets only.

FIG. 5E illustrates an embodiment in which the pillars are flexible. Theflexibility of a pillar may depend on one or more of pillar length,pillar diameter, the pillar material and the way in which the pillar ismade. In the embodiment of FIG. 5E the core is a multifaceted corehaving an irregular shape, although it will be appreciated that a corecarrying flexible pillars may have any particle core shape as describedherein.

A particle core with higher aspect ratio can increase the number ofconnections of the pillared particle with other elements in thecomposite electrode layer and/or the current collector and therebyimprove the electrode conductivity. This connectivity may be furtherincreased if one or more surfaces or part of one surface of the corehave no pillars on them.

A particle core with a high aspect ratio can enable a higher PMF or PVFvalue to be obtained whilst providing a pillared particle with at leastone dimension that is less than 10 microns.

If the pillared particle is made by etching a silicon starting materialthen a higher aspect ratio core can increase the manufacturing yield ofthe pillared particle in terms of the ratio of mass of the pillaredparticles produced relative to the mass of the starting material,compared to the yield with spheroidal starting materials. When anetching process is used, an increase in yield relates to a decrease inthe PMF or PVF value but this potential reduction in the maximumlithiation capacity of the pillared particle can be offset if theparticle core is made thin enough so that it can be lithiated to ahigher degree without pulverisation.

Preferably the particle core has a smallest dimension of at least 0.2μm, more preferably at least 0.5 μm.

If the particle core comprises an electroactive material, for examplethe core is silicon-comprising, then the core preferably has at leastone dimension less than 6 μm, more preferably less than 5 μm, mostpreferably less than 4 μm and especially less than 3 μm.

A smaller core dimension enables higher lithiation of the activematerial in the core without risk of cracking the core, increasing thepotential capacity of the pillared particle. It can also increase theattainable charge rate for high capacity anodes as the diffusion lengthfor metal ions is reduced.

It is preferable that the pillared particles have a low resistivity—thiswill increase the conductivity of composites containing them and improvethe cycling performance and charge rate of a metal ion battery. Somehigh capacity electroactive materials such as silicon have a relativelyhigh resistivity compared to that of lower capacity electroactivematerials such as graphite or non active metallic materials such ascopper, however with good electrode design, pillared particles withmedium range resistivity values can be used. Preferably the pillaredparticle has a resistivity of no more than 1000 Ωcm, more preferably nomore than 100 Ωcm, most preferably no more than 10 Ωcm, especially nomore than 1 Ωcm. The pillared particle may have a resistivity of atleast 1×10⁻⁵ Ωcm, for example at least 1×10⁻⁴ Ωcm or at least 5×10⁻⁴Ωcm.

The pillars preferably have a resistivity of no more than 100 Ωcm, morepreferably no more than 10 Ωcm, especially no more than 1 Ωcm. Thepillars may have a resistivity of at least 1×10⁻⁴ Ωcm, for example atleast 1×10⁻³ Ωcm or at least 1×10⁻² Ωcm.

When the particle core comprises electroactive material, it preferablyhas a resistivity of no more than 100 Ωcm, more preferably no more than10 Ωcm, especially no more than 1 Ωcm. A particle core comprisingelectroactive material may have a resistivity of at least 1×10⁻⁴ Ωcm,for example at least 1×10⁻³ Ωcm or at least 1×10⁻² Ωcm.

When the particle core does not comprise an electroactive material, itpreferably has a resistivity of no more than 10 Ωcm, more preferably nomore than 1 Ωcm, most preferably no more than 0.1 Ωcm and especially nomore than 0.01 Ωcm. When the particle core is not electroactive it isparticularly preferable that it has a resistivity of less than 5×10⁻³Ωcm

Pillar Length

FIGS. 4A and 4B illustrate etching of a starting material to producepillared particles. In this example, both the starting material 401 andthe pillared particle core 405 are substantially spherical for ease ofrepresentation, however it will be understood that both the startingmaterial and the pillared particle core may be of any shape.

In FIG. 4A, a starting material is etched to produce pillars 407 oflength L1. In FIG. 4B, a starting material 401 is etched to produceshorter pillars 407 of length L2. The longer pillars of FIG. 4A resultin a pillared particle with a higher PMF and may provide for highercapacity per unit mass of active material to insert lithium than theshorter pillars of FIG. 4B. The longer pillars of FIG. 4A also provide apillared particle with a larger specific surface area promoting contactof the electrolyte with the surface of the active material. However, inan etching process the yield in terms of the ratio of mass of thepillared particles produced relative to the mass of the startingmaterial will reduce as pillar length increases and may increase thecost of manufacturing electrode material. In addition, the higherspecific surface area of the pillared particle in FIG. 4A may increasethe amount of SEI layer formed in the electrode and may reduce thepotential number of charge/discharge cycles that may be achieved.

Providing pillared particles in which all dimensions are less than 10microns may limit the maximum length of the pillars, however it iseasier to form a composite electrode layer with a uniform thickness anda uniform distribution of pillared particles within the composite and toachieve a suitable density of the composite.

Additionally, as the charge capacity of silicon material is much largerthan graphite materials, when a cell comprises a composite anode layerwhere a significant proportion of the active material is pillaredparticles (for example where at least 20 wt % of the active material ispillared particles) then balancing the capacity of the anode to thecathode in the cell may mean that the anode layer must be made thin, forexample less than 30 μm thick. In this respect, using pillared particleswith at least one dimension less than 10 μm makes it easier tomanufacture such thin layers with minimal variations in thickness.

Furthermore, if the particle core comprises electroactive material, theability to stably lithiate and delithiate a higher volume fraction ofthe smaller core of a small pillared particle may at least partiallyoffset any reduction in capacity from shorter pillars.

The average pillar length is preferably less than 5 microns, and may bein the range of 0.5-5 microns. However, if pillars are provided on onlyone of two opposing surfaces of a pillared particle then the averagelength may be longer, optionally less than 8 microns.

Applications

The pillared particles described herein may be used as an activecomponent of an electrode, preferably an anode or negative electrode, ofa metal ion battery, preferably a lithium ion battery, having astructure as described with reference to FIG. 1.

The pillars of the pillared particles may be detached to form a fibrethat may likewise be used as a component of the anode of a lithium ionbattery. The silicon fibres can be made by detaching the pillars from apillared particle by one or more of scraping, agitating (especially byultrasonic vibration) or chemical etching.

A powder consisting essentially of the pillared particles may beprovided, for example by any of the aforementioned processes. Thispowder may be mixed with other materials to form a composition suitablefor use in forming the anode of a metal ion battery.

Other materials of this composition may include, without limitation, oneor more of:

a solvent or solvent mixture for forming a slurry containing thepillared particles (as will be understood by the skilled person, thesolvent or solvent mixture does not dissolve the pillared particles, andthe term “solvent” as used herein should be construed accordingly);other active materials; conductive, non-active materials, for exampleconductive, non-active carbon fibres; binders; viscosity adjusters;fillers; cross-linking accelerators; coupling agents and adhesiveaccelerators.

The pillared particles may be used as the only active component of ananode, or may be used in combination with one or more other activecomponents. In one embodiment, the pillars of the pillared particles,and optionally the core, are silicon, and the pillared particles aremixed with an active component formed from another material, for examplegraphite.

An active graphite electrode may provide for a larger number ofcharge/discharge cycles without significant loss of capacity than anactive silicon electrode, whereas a silicon electrode may provide for ahigher capacity than a graphite electrode. Accordingly, a composition ofa silicon-containing active material and a graphite active material mayprovide a lithium ion battery with the advantages of both high capacityand a large number of charge/discharge cycles. The use of pillaredparticles having at least one dimension less than 10 microns asdescribed herein may be particularly advantageous in view of the greatercapacity per volume or capacity per mass of such pillared particles ascompared to larger pillared particles.

A composition of graphite and a pillared particle comprising silicon maycontain at least 5 weight % silicon, optionally at least 10 weight %silicon.

In order to form the anode of a battery a slurry containing the pillaredparticles in a solvent or solvent mixture may be deposited on an anodecurrent collector formed from a conductive material, for example copper,followed by evaporation of the solvent(s). The slurry may contain abinder material and any other active materials to be used in the anode.Exemplary binders include polymer binders such as polyacrylic acid(PAA), polyimide (PI), polyvinylalcohol (PVA) and polyvinylidenefluoride (PVDF), carboxymethylcellulose (CMC), (styrene-butadiene rubber(SBR) and metal ion salts thereof. A binder may also be a mixture of oneor more polymers. Other materials that may be provided in the slurryinclude, without limitation, a viscosity adjuster, a filler, across-linking accelerator, a coupling agent and an adhesive accelerator.The component materials of the composite are suitably mixed together toform a homogeneous electrode composition that can be applied as acoating to a substrate or current collector to form a compositeelectrode layer adhered to said substrate of current collector.

The composite electrode containing pillared particles may be porous toenable wetting of the active material by the electrolyte and to providespace to accommodate the expansion of active material during charge andprevent swelling of the electrode. The composite porosity may be definedas the total volume of pores, voids and empty spaces in the compositeelectrode in the uncharged state before any electrolyte is added to orcontacted with the composite electrode, as a percentage of the totalvolume occupied by the composite material layer. It may be measured by,for example, mercury or nitrogen porosimetry.

However if the porosity is too high the mechanical integrity of theelectrode may be affected and the charge capacity per unit volume (ormass) may be reduced A suitable level of porosity may depend on severalfactors including but not limited to composition, particle size, type ofelectrolyte/binder, layer thickness, cell type/design. At least some ofthe porosity will be provided by the void space between the pillars ofthe pillared particles. Preferably the porosity of the composite in theuncharged state is at least 10%, more preferably at least 20% andespecially 30%. Preferably the porosity of the composite in theuncharged state is no more than 80%, more preferably no more than 60%.

Preferably the porosity of the composite material is at least twice theratio of the volume of the pillars of the pillared particles containedin the composite as a percentage of the total volume occupied by thecomposite material layer. This applies in particular where the surfaceof the core is not an active material, or the core expands by no morethan 10% upon full lithiation, in which case the minimum pore volume ispreferably 2× the volume of the pillars. Preferably in this case, themaximum pore volume is 4× the volume of the pillars+1.1× core volume.

If the composite material contains pillared particles with particlecores comprising electroactive material, preferably an electroactivematerial that expands by more than 10% on full lithiation, the porositymay be higher to further accommodate the expansion of the particle coreswhen they are lithiated. In this case a suitable minimum compositeporosity may be given by the sum of the volume of pillars multiplied bytwo and the volume of particle cores multiplied by 1.2, as a percentageof the total volume of the composite material layer.

Preferably, maximum pore volume provided by the pillaredparticles=4×(pillar volume+core volume)=4×volume of pillared particles.

The appropriate minimum or maximum composite porosity is calculated fromthe aforementioned pore volumes by dividing the pore volumes by thetotal volume of the composite layer×100%.

Porosity is of the composite as a whole, which may include, withoutlimitation, porosity provided by space between pillars, and spacebetween particles of the composite electrode.

Porosity used in a battery is a balance between high porosity to enablegood surface contact between the electrolyte of the electroactivematerials and to provide a buffer space to minimise overall expansion ofthe electrode during charging and a porosity that is low enough toprovide good cohesion of the composite electrode and good adhesion tothe anode current collector, together with an appropriate density ofelectroactive material per unit volume of electrode which affects theoverall volumetric capacity/rating of the battery. Pillared particles asdescribed herein provide an effective way of introducing porosity into acomposite electrode, with an optimal level of porosity which isparticularly beneficial for silicon-containing anodes due to the highspecific capacity of silicon and associated high degree of expansion ofsilicon upon charging.

The anode composite material layer may be any suitable thickness. Thepillared particles of this invention are especially advantageous formaking composite layers with an average thickness of less than 60 μm oreven less than 30 μm (not including the thickness of the currentcollector). Preferably the composite layer thickness is at least 10 μmthick, more preferably at least 12 μm thick. The anode may comprise acomposite layer deposited/attached on one or both sides of the currentcollector.

Examples of suitable cathode materials include LiCoO₂,LiCo_(0.99)Al_(0.01)O₂, LiNiO₂, LiMnO₂, LiCo_(0.5)Ni_(0.5)O₂,LiCo_(0.7)Ni_(0.3)O₂, LiCo_(0.8)Ni_(0.2)O₂, LiCo_(0.82)Ni_(0.18)O₂,LiCo_(0.8)Ni_(0.15)Al_(0.05)O₂, LiNi_(0.4)Co_(0.3)Mn_(0.3)O₂ andLiNi_(0.33)Co_(0.33)Mn_(0.34)O₂, LiFePO₄, LiVPO₄F, LiMn₂O₄,LiNi_(0.8)Co_(0.15)Al_(0.05)O₂, LiNi_(0.5)Co_(0.2)Mn_(0.03)O₂,xLi₂MnO₃(1−x)LiMO₂, Li₂FeS₂, vanadium oxides and sulphur basedcompounds. The cathode current collector is generally of a thickness ofbetween 3 to 500 μm. Examples of materials that can be used as thecathode current collector include aluminium, stainless steel, nickel,titanium and sintered carbon.

The electrolyte is suitably a non-aqueous electrolyte containing alithium salt and may include, without limitation, non-aqueouselectrolytic solutions, solid electrolytes and inorganic solidelectrolytes. Examples of non-aqueous electrolyte solutions that can beused include non-protic organic solvents such as N-methylpyrrolidone,propylene carbonate, ethylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, vinyl carbonate, vinylene carbonate,vinylethylene carbonate, butylene carbonate, dimethyl carbonate, diethylcarbonate, gamma butyro lactone, 1,2-dimethoxy ethane, 2-methyltetrahydrofuran, dimethylsulphoxide, 1,3-dioxolane, formamide,dimethylformamide, acetonitrile, nitromethane, methylformate, methylacetate, phosphoric acid trimester, trimethoxy methane, sulpholane,methyl sulpholane and 1,3-dimethyl-2-imidazolidione.

Examples of organic solid electrolytes include polyethylene derivativespolyethyleneoxide derivatives, polypropylene oxide derivatives,phosphoric acid ester polymers, polyester sulphide, polyvinyl alcohols,polyvinylidine fluoride and polymers containing ionic dissociationgroups.

Examples of inorganic solid electrolytes include nitrides, halides andsulphides of lithium salts such as Li₅NI₂, Li₃N, LiI, LiSiO₄, Li₂SiS₃,Li₄SiO₄, LiOH and Li₃PO₄.

The lithium salt (or mixture of salts) is suitably soluble in the chosensolvent or mixture of solvents. Examples of suitable lithium saltsinclude LiCl, LiBr, LiI, LiClO₄, LiBF₄, LiB₁₀C₂₀, LiPF₆, LiCF₃SO₃,LiAsF₆, LiSbF₆, LiAlCl₄, CH₃SO₃Li, Lithium bis(oxalto)borate (LiBOB) andCF₃SO₃Li.

Where the electrolyte is a non-aqueous organic solution, the battery isprovided with a separator interposed between the anode and the cathode.The separator is typically formed of an insulating material having highion permeability and high mechanical strength. The separator typicallyhas a pore diameter of between 0.01 and 100 μm and a thickness ofbetween 5 and 300 μm. Examples of suitable electrode separators includea micro-porous polyethylene film.

In addition to lithium ion batteries, pillared particles as describedherein may be used in solar cells (including solar capacitors),capacitors, filters, fuel cells, detectors and sensors.

EXAMPLES

Pillared Particle Formation

Three powder samples of pillared particles, designated A, B and C, wereprepared by etching silicon granule starting materials with the D10, D50and D90 size distribution values listed in Table 1. The particle sizeswere measured using a MasterSizer™ 2000 version 5.60 with a waterdispersant. Etching to produce silicon pillared particles was performedas described in WO 20101040985. Table 1 lists the BET, PMF and averagepillar lengths for each pillared particle sample. Average pillardiameters for all three samples were in the range of 50-150 nm. A meanaverage pillar length may be determined from SEM measurements of aplurality of pillar lengths. Typical pillar densities were 25-40%.

TABLE 1 Powder A Powder B Powder C Si purity   99.8 wt %   99.8 wt %99.95 wt %  BET of starting 0.34 0.67 0.98 material (m2/g) MastersizerD10 14.7 μm  8.2 μm 6.0 μm (starting material) Mastersizer D50 23.7 μm13.5 μm 9.9 μm (starting material) Mastersizer D90 37.6 μm 21.9 μm 15.9μm  (starting material) BET of pillared 45 32.9 4.5 particles (m2/g)Average PMF 30-35% 34% 24% (=PVF) Average pillar  2-3 μm 1.8-2 μm 1.6-1.8 μm   lengths (μm) No. of cycles at 250 330 >390 1200 mAh/gbefore fading to 90%

FIG. 6 illustrates a size distribution of the spherical equivalentvolume diameters of the starting material particles for Powder C. TheD50 value is 9.9 microns showing that 50% of the volume (and 50% of themass) of the starting material particles have a spherical equivalentvolume diameter of less than or equal to 9.9 microns.

FIG. 7 is a SEM photograph of pillared particles of Powder C (Example2). It can be seen that the powder includes particles having at leastone dimension that is less than 10 microns, as well as particles havinga dimension of 10 microns or more, although the size distribution of theparticles is such that at least 50% of the particles have a dimension ofless than 10 microns.

FIG. 8 is a further SEM photograph of the pillared particles or Powder C(Example 2). The pillared particles include pillars having a length ofabout 2 microns.

The Dn size distribution of the starting material and the product may besubstantially the same. Metallurgical grade silicon starting powderavailable from Elkem (Silgrain HQ), was etched using metal assistedetching as described above under “Pillared Particle Formation”. Thestarting material had D10, D50 and D90 values of 2.8, 4.6 and 7.9microns respectively. The product, referred to as Powder Product D, hadD10, D50 and D90 values of 2.8, 4.6 and 7.6 microns respectively bothmeasured using a Malvern Mastersizer, indicating that the volumedistribution of particles with effective spherical diameters of a sphereencompassing the core and pillars of the pillared particles may remainsubstantially the same as the volume distribution of effective sphericaldiameters of a sphere encompassing the starting material.

The volume distribution of the pillared particle product is illustratedin FIG. 9.

Electrode and Cell Fabrication

Anode Preparation

The desired amount of pillared particle composition was added to aconductive carbon mixture that had been bead milled in deionised water.The resulting mixture was then processed using an IKA overhead stirrerat 1200 rpm for around 3 hours. To this mixture, the desired amount ofbinder in solvent or water was added. The overall mix was finallyprocessed using a Thinky™ mixer for around 15 minutes. Viscosity of themix was typically 500-3000 mPas at 20 rpm.

Product Powder A comprised pillared particles produced from a startingmaterial of silicon particles with average dimensions of 15-25 μm and aD10 value of 14.7 μm. Pillared particles produced from this startingmaterial had a pillar volume fraction of 30-35%, a BET value of 45 m²/g,pillars of average length 2-3 μm and diameters of 50-150 nm. Compositeanode D was made with 70% by mass of these pillared particles, 15%Na-PAA binder and 15% conductive carbon by mass.

Product Powder B (Example 1) comprised pillared particles produced froma starting material of silicon particles including particles with atleast one dimension less than 10 μm and a D10 value of 8.2 μm. Pillaredparticles produced from this starting material had a pillar volumefraction of 34%, a BET value of 32.9 m²/g, pillars of average length1.8-2 μm and diameters of 50-150 nm. Composite anode E was made with 70%by mass of these pillared particles, 15% Na-PAA binder and 15%conductive carbon by mass.

Product Powder C (Example 2) comprised pillared particles produced froma starting material of silicon particles including particles with atleast one dimension less than 10 μm and a D10 value of 6 μm. Pillaredparticles produced from this starting material had a pillar volumefraction of 24%, a BET value of 4.5 m²/g, pillars of average length1.6-1.8 μm and diameters of 50-150 nm. Composite anode F was made with70% by mass of these pillared particles, 15% Na-PAA binder and 15%conductive carbon by mass.

The composite anode mixture was applied to a 10 μm thick copper foil(current collector) using a doctor-blade technique to give a 20-35 μmthick coating layer (coat weight of 14-15 gsm). The resulting electrodewas then allowed to dry.

Cathode Preparation

The cathode material used in the test cells was a commercially availablelithium mixed metal oxide (MMO) electrode material (e.g.Li_(1+x)Ni_(0.8)Co_(0.15)Al_(0.05)O₂) on a stainless steel currentcollector.

Electrolyte

The electrolyte used in all cells was lithium hexafluorophosphate,dissolved in a mixture of ethylene carbonate and ethyl methyl carbonate(in the ratio 3:7 by volume) and containing 15 wt % fluorethylenecarbonate (FEC), and 3 wt % vinylene carbonate (VC) additives. Theelectrolyte was also saturated with dissolved CO₂ gas before beingplaced in the cell.

Cell Construction and Testing

Test cells were made using composite anodes D, E and F as follows:

-   -   Anode and cathode discs of 12 mm diameter were prepared and        dried over night under vacuum.    -   The anode disc was placed in a 2-electrode cell fabricated from        Swagelok® fittings.    -   Two pieces of Tonen® separator of diameter 12.8 mm and 16 um        thick were placed over the anode disc.    -   40 μl of electrolyte was added to the cell.    -   The cathode disc was placed over the wetted separator to        complete the cell.    -   A plunger of 12 mm diameter containing a spring was then placed        over the cathode and finally the cell was hermetically sealed.        The spring pressure maintained an intimate interface between the        electrodes and the electrolyte.    -   The electrolyte was allowed to soak into the electrodes for 30        minutes.

Once assembled, each cell was connected to an Arbin™ battery cyclingrig, and tested on continuous CC charge and discharge cycles as follows.For the initial cycle, the cell was charged to a maximum capacity of2000 mAh per gram of silicon or until the voltage decreases to 0.005 V,whichever occurs first. After five minutes rest, the cell was thendischarged to a voltage of 1.0 V vs lithium. The cell is then rested for30 minutes. The cell is subsequently charged/discharged at a C/5 rate bycharging the cell to either 1200 mAh per gram of silicon (or approx.1300 mAh/g for the cell with composite anode E) or 0.005 V, whicheveroccurs first, resting for 5 minutes and then a constant currentdischarge to 1.0 V vs lithium and resting for 30 minutes beforecommencing the next charge.

FIG. 10 plots the specific discharge capacity vs number ofcharge/discharge cycles for cells containing composite anodes D(containing the powder of relatively large particles), E and F. It canbe seen that composite anodes E and F comprising pillared particles withat least one dimension less than 10 μm and a D10 value of less than 10μm provide more charge/discharge cycles than composite anode Dcomprising larger pillared particles.

Example 2

Metallurgical grade silicon powders with different size distributionsavailable from Elkem as Silgrain HQ were etched to form pillaredparticles using the method described above under “Pillared ParticleFormation”. Cells were formed substantially as described above under“Electrode and cell fabrication”. Details of the materials and cells areset out in Table 2.

TABLE 2 Discharge D10/50/90 of capacity at 2 C Powder the product BET asa % of Product (microns) (m²/g) PMF (%) BET/PMF capacity at C/5 E13/20.6/32.1 32 20 1.6   75% F 6.6/11.4/19.6 10 24 0.42 86.3%

Powder E has a D10 value that is greater than 10 microns, whereas powderF has a D10 value below 10 microns.

FIG. 11 is a SEM image of pillars of a pillared particle formed byetching the starting material used to form Powder F.

The discharge capacity at 2C (full discharge in 30 minutes) as apercentage of the discharge capacity at C/5 (full discharge over aperiod of 5 hours) is higher for the smaller particles of Powder E thanfor Powder E.

Cells formed from powders E and F were each charged at a rate C/2 tofull capacity and discharged at rates C/5, C/2, C and 2C, wherein C isthe rate at which full charge or discharge capacity is reached in a timeof 60 minutes. With reference to FIG. 12 (Powder F cell) and FIG. 13(Powder E cell), the discharge capacity/charge capacity ratio is higherfor the cell containing the smaller Powder F, particularly at higherdischarge rates. This demonstrates that the cell comprising the smallerpillared particles with a BET/PMF ratio less than 1.5 have a better rateperformance than the cell comprising the larger particles.

Example 3

In order to assess the effect of silicon particle size on cellexpansion, half cells were made having an composite electrode containingsilicon powder, lithium foil as the counter electrode and a liquidelectrolyte. The silicon containing composite electrodes were made asdescribed above. The electrolyte was as described in example 1. Theincrease in the thickness of the silicon-containing composite electrodelayer (excluding the current collector) was measured as the cell wascharged (first cycle) with an El-Cell® Electrochemical DilatometerECD-nano placed inside a temperature controlled chamber at 20° C.

The electrodes contained 70:14:16 weight % silicon particles:NaPAAbinder:carbon additives. The carbon additives were a mixture of graphiteflakes and conductive carbon such as carbon black. Each electrodecontained different silicon material as described below in Table 3:Powder products G, and H were pillared particles made by etching siliconstarting material powders as used for powders A and B respectively.Powder product J was pillared particles made by etching the samestarting material as used for Powder Product D. Powders Hb and Jb werenon-pillared particles, being samples of the same starting materialsused for Powder Products H and J respectively, but remaining unetched.

Results are shown in Table 3. Capacities are per gram of silicon.

TABLE 3 Electrode % expansion in electrode Powder D10/50/90 BET PMF BET/coating thickness Product of product (m2/g) (%) PMF porosity at 1500mAh/g at 2,000 mAh/g at 3,000 mAh/g G 14.5/23.4/ 20.2 15% 1.35 61% 39 66183 37.6 H 5.9/10.5/18.2 49.9 33% 1.5 64% 32 52 136 J 2.8/4.6/7.6 36.727% 1.4 57% 25 41 108 Hb 8.2/13.5/21.9 0.67 0 43% 125 208 420 Jb2.8/4.6/7.1 5.0 0 44% 66 122 310

FIG. 14 is a SEM image of a pillared particle powder formed by etchingthe starting material used to form Powder J, showing this particularlysmall pillared particle material.

As can be seen in FIG. 14, not all faces of all particle cores carrypillars, and it will be understood that pillared particles as describedherein may each have a surface with a plurality of faces and all, someor only one of these faces may carry pillars. It will further beunderstood that the powders may contain particles on which no pillarshave formed, although preferably at least 50%, at least 75% or at least90% of particles of a powder of pillared particles (as observed by SEM)carry pillars.

With reference to FIG. 15 and Table 3, the thickness of the electrodescontaining the starting materials expands to a much greater degree thanthose containing pillared products formed from the starting material.Moreover, Powders H and J, which both have a D10 value below 10 microns,expand less than Powder G, which has a D10 value above 10 microns.

Example 4

The starting material is silicon flakes of average thickness 5 μm andwidth/length dimensions of about 10 μm, made from metallurgical gradesilicon or doped silicon wafer. The flakes are etched to produce siliconpillars (nanowires) on both major surfaces of each flake, the pillarshaving an average diameter of 80 nm, length of 1.5-2 μm and a averagepillar density of 10 to 40% or 20 to 40%. The remaining particle corehas a thickness of 1-2 μm.

The pillared particles thus produced have a PVF (=PMF) value of 25-70%,a BET value of 5-30 m²/g and an etching yield of 10-40%. The low valueof the particle core thickness enables substantial lithiation of thecore without fracture, enhancing the overall specific charge capacity ofthe particle and the high aspect ratio shape enables a good balance tobe obtained between Pillar Volume Fraction and Yield. In particular, ayield of >30% combined with a PVF (and PMF) of >30% is achieved forflakes with an average pillar density of 40% and pillars of length 1.5μm. A plurality of such pillared particles may be comprise 60-80 wt % ofa composite anode, with 8-20 wt % of a polymer binder, 0-20 wt %graphite particles and 5-20 wt % conductive (non active) carbonadditives, such that the sum of the component percentages adds up to100%. Because such a composite has a very high capacity then a thincomposite layer, e.g. less than 25 μm, may be necessary to match theanode to the cathode (for example the thickness of the cathode layer maybe determined such that it has discharge capacity that is 10-25% inexcess of that of the anode layer to increase the number of cycles thatcan be achieved). The small size of the pillared particles with at leastone dimension less than 10 μm makes it easier to coat such a thin anodelayer on the current collector.

To further demonstrate the benefit in yield gained when the pillaredparticles are made by etching silicon flakes, Table 4 below gives themaximum yield when etching 5 μm thick square silicon flakes of differentaspect ratios (the length of the sides of the square divided by 5 μm) toproduce particles comprising pillars of height 1.5 μm and diameter 80 nmon the top and bottom surfaces at a density sufficient to produce a PMFof 25%. The BET values will not be strongly affected by the aspect ratio(e.g. for solid pillars with smooth surfaces the BET values for examples2a to 2d are 5.9 to 6.4, giving a BET/PMF ratio of 0.24-0.27. Rough orporous pillar surfaces will increase the BET accordingly). However asthe aspect ratio is increased, the yield increases significantly.Preferably the aspect ratio is at least 1.5:1 with a pillar density ofat least 10%.

TABLE 4 Example Aspect ratio Pillar density (%) Yield (%) 2a 1.5 12 192b 2 14 26 2c 3 17 34 2d 1 7 8.5

FIG. 16A illustrates a silicon pillared particle having a thin, highaspect ratio core, a BET value of 56 m2/g, a PMF of 33% and a BET/PMFratio of 1.7.

FIG. 16B illustrates a silicon pillared particle having a thin, highaspect ratio core, a BET value of 13 m2/g, a PMF of 21% and a BET/PMFratio of 0.62. Pillar length is about 1.5 microns.

Example 5

The pillared particles described in example 2 may also be used as a highcapacity active additive to a graphite based anode composite. Whilst thecharge capacity per unit volume of such a cell may be less than onewhere the majority of the anode active material is silicon comprising,for certain cell designs it may make matching of the electrodes easiersince thicker coatings can be used for the composite anode. When siliconcomprising particles are used as additives in a composite where themajority of the active material is graphite, because of the differentelectrochemical potentials of graphite and silicon, the siliconcomprising particles may be fully lithiated before lithiation of thegraphite is initiated, therefore it is advantageous if both the core andthe pillars of the pillared particles can be substantially fullylithiated over many cycles without degradation. A plurality of pillaredparticles described in example 2 may be comprise 5-25 wt % of acomposite anode, with 8-20 wt % of a polymer binder, 50-80 wt % graphiteand/or graphene particles and 5-20 wt % conductive (non active) carbonadditives, such that the sum of the component percentages adds up to100%.

Example 6

The starting material is graphite and/or conductive carbon particleswith at least one smaller dimension less than 8 μm, preferably around 5μm and dimensions orthogonal to the smaller dimension of no more than 20μm. The starting material has a D10 value less than 10 μm as measured ona powder sample dispersed in water by a Malvern Mastersizer system.

The graphite particles are placed on a substrate and coated with Au, Nior Sn catalyst particles. Silicon nanowires are grown on the exposedsurfaces of the graphite at points where the catalyst particles arelocated via CVD-assisted VLS process (for example PECVD, LPCVD or PPCVDsystems may be used). The silicon nanowires have diameters of 30-60 nmand lengths of at least 4 μm, for example between 4 and 10 μm long. Thesurface of the graphite/carbon particles in contact with the substratemay not be covered with pillars. Alternatively the starting material canbe coated with catalyst particles from solution and the nanowires grownin a fluidised bed reactor to form pillars on all surfaces of theparticle core. The pillared particles are removed from the substrate (orreactor) for testing or added to a slurry for making a composite anodelayer. The pillared particles have a PVF value of 5-15% and the BETvalue is less than 30 m²/g, the average pillar density is 0.5-5%. Acomposite anode material comprises a mixture of the pillared particles,a binder and additional conductive additives (e.g. carbon) withrespective mass percentages of 65-85 wt % pillared particles, 8-15 wt %binder and 5-20 wt % conductive additives, such that the sum of thecomponent percentages adds up to 100%. Such a mix would comprise 3-13 wt% of active silicon material and 55-80 wt % of active graphite material.Alternatively, some of the pillared particles could be replaced by baregraphite particles whilst still maintaining the mass percentage ofsilicon and graphite within these ranges.

Example 7

The pillared particles are made as described in example 5 except some orall of the graphite particles are replaced by graphene particlescomprising at least 4 graphene sheets. The graphene particles have ahigh aspect ratio with a thickness less than 1 μm and length/widthdimensions less than 15 μm. Preferably the BET value of the grapheneparticles is less than 200 m²/g, more preferably less than 100 m²/g. Aswell as providing a significantly higher conductivity (lowerresistivity), the thinner core means that longer pillars can be grownwhilst maintaining the D10 value of less than 10 μm and increasing thePVF value (i.e. increasing the specific charge capacity), for example inexcess of 40%. However the higher BET value of the graphene corepillared particles may increase the first cycle loss and it ispreferable to mix graphite core pillared particles with graphene corepillared particles in the ratio 9:1 to 7:3. The composite anode mixcomprises 8-15 wt % binder, 5-20 wt % conductive additives, 5-20 wt %silicon and 50-80 wt % graphite and/or graphene, such that the sum ofthe component percentages adds up to 100%.

Example 8

Pillared particles were prepared as described above under “PillaredParticle Formation” using powders available from Elkem of Norway. Cellswere prepared as described above under “Cell Construction and Testing”.

Properties of the pillared particles are set out in Tables 5A and 5B

TABLE 5A Pillared particle BET PMF BET/ Cycles to 80% Product D10/50/90(m²/g) (%) PMF capacity Powder K 2.6/4.5/8.2 25.2 30 0.84 319 Powder L2.8/4.6/7.6 36.7 30 1.22 361 Powder M 6.5/10.7/17.5 26.9 28 0.96 279Powder N 6.3/10.9/18.7 14.6 21 0.7 289 Powder O 2.5/4.7/9.6 40.3 0 ∞ 184(Comparative Example)

TABLE 5B Cycles to Pillared 4.3 V end Average eff particle BET of chargeFCL % (cycles 3- Product D10/50/90 (m²/g) PMF BET/PMF voltage (%) end)Powder P 6.3/10.9/18.9 30 22 1.4 256 25 99.47 Powder Q 6.2/11.1/19.9 5114 3.6 127 41 99.37

In the above table, FCL is first cycle loss, which represents theirreversible loss of lithium during the first charge/discharge cycle asa percentage for the charge capacity in the first cycle. Some of thelithium loss can be attributed to the cathode (positive electrode)whilst the remainder results from the formation of the SEI layer on thesurface of the electroactive material in the negative electrode duringthe first charge cycle.

Cells containing Powders K-O were cycled between fixed upper and lowercell voltage limits corresponding to a charging capacity ofapproximately 900 mAh/g of silicon.

Powders K and L are smaller than powders M and N, and maintain acapacity above 80% of a starting capacity for more cycles than largerpowders M and N.

For the purpose of comparison, a powder M was prepared in which thedensity of silver nucleation was controlled to form interconnected poresextending into a surface of the silicon starting material rather thanpillars extending from an etched surface of the silicon. FIG. 17 is aSEM image of Powder M, showing pores extending into the surface of thesilicon rather than discrete pillars extending from the surface of asilicon core on the majority of core surfaces.

It can be seen that the performance of Powder M is significantly worsethan any of powders K-N containing pillared particles.

Cells containing powders P and Q were cycled at a constant capacity of1200 mAh/g of silicon until the end of discharge voltage reached 4.3V,at which point a capacity of 1200 mAh/g can no longer be maintained.With reference to powders P and Q, it can be seen that powder P, havinga BET/PMF ratio of 1.4, can be cycled at a capacity of 1200 mAh/g fortwice the number of cycles as powder Q, which has a BET/PMF ratio above3.

With reference to FIG. 18, the present inventors have found a linearrelationship between first cycle loss and BET for etched siliconpillared particles. Without wishing to be bound by any theory, it isbelieved that if the surface area per unit mass is too large then thecharge capacity per unit mass and/or cycle life may be reduced throughexcessive formation of oxide and/or SEI layer on the surface of theactive material. The present inventors have found that the non-linearrelationship between BET and PMF is such that the preferred BET/PMFratio below 3.

Example 9

Pillared particles, composite electrodes containing the particles andcells containing the electrodes were prepared and tested as in Example8, except that the cells were lithiated and delithiated to maximumcharge/discharge capacity at each cycle rather than between fixed upperand lower voltage limits or at fixed charge capacities (as in previousexamples) in order to maximise mechanical stress of the active siliconas both the cores and pillars are fully lithiated.

Results are provided in Table 6.

TABLE 6 Cycles to Cycles to 60% 70% Initial Av. Eff initial initialcapacity from capacity capacity (2^(nd) 3^(rd) Pillar BET PMF BET/(2^(nd) 2^(nd) cycle) FCL cycle to Product D10/50/90 (m²/g) (%) PMFcycle) cycle) mAh/g (%) end O 2.5/4.7/9.6 40.3 0 ∞ 60 43 3660 14 99.06(Comp. Example) S 2.8/4.6/7.8 14 37 0.64 215 133 3706 13 99.40 T2.8/4.6/7.4 18.9 42 0.7 >250 84 3341 12 99.17 U 2.6/4.6/7.3 23 41 0.88272 163 2263 17 98.65 V 2.6/4.3/6.9 16.9 29 0.83 226 118 2968 13 99.19 W6.9/11.3/18.4 19.3 33 0.92 130 38 2919 10 99.36

For the purpose of comparison, Powder O was used. As described above,the density of silver nucleation was controlled to form Powder Ocontaining particles of interconnected pores extending into a surface ofthe silicon starting material rather than pillars extending from anetched surface of the silicon.

FIG. 19 is a SEM image of Powder S, showing pillars extending from theparticle core.

As with Example 6, the porous, non-pillared particles (Powder O) providesubstantially worse performance than the pillared particle.

Although the number of cycles to fall to 70% initial capacity for PowderW is smaller than for non-pillared powder O, the number of cycles tofall to 60% capacity is much higher for pillared Powder W. Withoutwishing to be bound by any theory, it is believed that this relativelyshort number of cycles to fall to 70% capacity is due to lithiation ofthe core of the relatively large particle of Powder W.

Example 10

Pillared particles were prepared and devices were prepared and tested asin Example 8 with the cells being cycled at a fixed capacity of 1200mAh/g silicon. Pillared particles having different BET/PMF ratios wereprepared from the same starting material as used for powder B.

As shown in Table 7, average efficiency over the third to fiftiethcharge-discharge cycles is higher, and first cycle capacity loss islower, for pillared powders Y and Z, which have a BET/PMF ratio below 3as compared to powder X which has a BET/PMF ratio above 3.

TABLE 7 Average efficiency Pillared BET Corrected over 3-50 product(m²/g) PMF (%) BET/PMF FCL (%) cycles X 75 24 3.125 21% 99.63% Y 38.4 221.74 17% 99.76% Z 9.3 23 0.4 11.5%   99.87%

Although the present invention has been described in terms of specificexemplary embodiments, it will be appreciated that variousmodifications, alterations and/or combinations of features disclosedherein will be apparent to those skilled in the art without departingfrom the scope of the invention as set forth in the following claims.

The invention claimed is:
 1. A powder comprising a plurality of pillared particles for use as an active component of a metal ion battery, the pillared particles comprising a particle core and a plurality of elongated structures extending from the particle core, wherein the elongated structures comprise at least one of silicon, tin, and germanium, wherein a BET value of the pillared particles is 1-200 m²/g, wherein a pillar mass fraction (PMF) of the pillared particles is in a range of 5-80%, wherein PMF=[(Total mass of elongated structures extending from the particle core)/(Total mass of pillared particle)]×100%, wherein an average density of the elongated structures extending from the particle core is in a range of 1-80%, wherein average density of the elongated structures is given by the formula A/(A+B)×100% wherein A is the area of a surface of the particle core occupied by elongated structures and B is the area of the surface that is unoccupied by elongated structures; and wherein a mean average thickness of the elongated structures is in a range of 10-250 nm.
 2. The powder according to claim 1, wherein the PMF is in a range of 10-80%.
 3. The powder according to claim 1, wherein the PMF is in a range of 20-60%.
 4. The powder according to claim 1, wherein the average density of the elongated structures extending from the particle core is in a range of 1-60%.
 5. The powder according to claim 1, wherein the average density of the elongated structures extending from the particle core is in a range of 10-50%.
 6. The powder according to claim 1, wherein the mean average thickness of the elongated structures is in a range of 10-150 nm.
 7. The powder according to claim 1, wherein the mean average thickness of the elongated structures is in a range of 10-80 nm.
 8. The powder according to claim 1, wherein the BET value of the pillared particles is 5-100 m²/g.
 9. The powder according to claim 1, wherein the BET value of the pillared particles is 5-100 m²/g, the average density of the elongated structures extending from the particle core is in a range of 1-60%, and the PMF is in a range of 20-60%.
 10. The powder according to claim 1, wherein the pillared particles have at least one dimension that is less than 10 microns.
 11. The powder according to claim 1, wherein at least 10% of a total volume of the powder is made up of particles having a particle size of no more than 10 microns.
 12. The powder according to claim 1, wherein the volume of the elongated structures is at least 20% of the total volume of the pillared particles.
 13. The powder according to claim 1, wherein an average length of the elongated structures is less than 10 microns.
 14. The powder according to claim 1, wherein an average length of the elongated structures is less than 5 microns.
 15. The powder according to claim 1, wherein the elongated structures comprise silicon.
 16. The powder according to claim 1, wherein the elongated structures do not comprise carbon.
 17. The powder according to claim 1, wherein the elongated structures are spaced apart from one another.
 18. The powder according to claim 1, wherein the particle core comprises an electroactive material comprising one or more of graphite, graphene, hard carbon, silicon, germanium, gallium, tin, aluminium, lead, indium, antimony, bismuth, oxides, nitrides or hydrides thereof, mixtures of these, mixtures or composite alloys containing these elements and chalcogenides and ceramics that are electrochemically active.
 19. The powder according to claim 1, wherein the particle core comprises carbon.
 20. The powder according to claim 1, wherein the particle core comprises a material selected from the group consisting of hard carbon, graphite, and graphene.
 21. The powder according to claim 1, wherein opposing surfaces of the pillared particles carry elongated structures.
 22. The powder according to claim 1, wherein only one of two opposing surfaces of the pillared particles carries elongated structures.
 23. The powder according to claim 1, wherein the pillared particles are substantially discrete from one another.
 24. A composition comprising a powder according to claim 1, and further comprising at least one of: (i) at least one further active component; (ii) at least one conductive, non-active component; (iii) a binder; and (iv) a solvent.
 25. A composite electrode comprising a powder according to claim 1, further comprising at least one of: (i) at least one further active component; (ii) at least one conductive, non-active component; and (iii) a binder. 